Would a diagnosis from a layman be legitimate if it follows the definition of the disorder? Assuming a layman with no formal training (person A) says "person B has disorder X". A should not make such diagnosis because they don't have the training. But if A does that because it fits the definition, then wouldn't their conclusion be correct? For example, I know that for a person to have borderline personality disorder they need to have at least 5 of 9 criteria. So if B does have that, then would A be fine to conclude that B has that disorder, in spite of the fact that A isn't trained?
Here are some basics on the diagnostic process:


  There are two main sources of disagreement in
  assessing psychiatric disorder or, as Feinstein
  (1985) wrote, 'two different phases of the
  observational process'. First, in the assessment
  of psychopathology; to transform ' the observed
  phenomena into certain raw elements of descriptive
  data'. Second, disagreement arises from
  the combining of detected psychopathology to
  yield diagnoses;' In the second phase, these raw
  descriptive elements are converted into a
  category of final expression' (Feinstein, 1985).
  Efforts have been made to standardize both
  these areas. [...]


And if this needs a quote, here's the obvious fact:


  Lay interviewers are unable to exercise expert
  clinical judgements in making decisions about
  the presence or absence of symptoms. [...]
  
  There are some particular problems in attempting
  to standardize assessments of psychiatric
  disorder. Lack of insight into mental
  illness, a problem when assessing people with
  psychotic illness especially, can affect responses
  to questions. The extreme example is the deluded
  patient who will say there is nothing wrong. In
  the training of a clinical psychiatrist much stress
  is put on obtaining historical details from
  informants other than the patient because of
  clinical evidence that those with severe mental
  illness can be misleading historians. For
  example, a subject with a delusion of persecution,
  may state that neighbours are deliberately
  making loud noises, or moving furniture in
  order to annoy. Such statements would lead
  most clinicians to investigate further by means
  of informants, though others might rely purely
  on clinical impressions from an interview.
  Attempts to standardize assessments of psychosis
  are more difficult and of a different nature
  from the problems of standardizing minor
  psychiatric disorder when insight is rarely
  impaired to an extent which affects the validity
  of self-report. A similar problem arises when
  attempting to standardize assessments of conversion
  hysteria or other physical disorders with
  a presumed psychological aetiology.


It's not a slam dunk that standazing the interviewing process will eliminate experience as a factor, for example


  There are also concerns that lay interviewers
  are an important source of bias when using the
  PSE. Lay interviewers tend to score more highly
  than psychiatrists and so estimate a higher
  prevalence of psychiatric disorder. For example,
  Sturt et al. (1981) found that the mean PSE
  scores for lay interviewers were nearly 30%
  higher (4-12 v. 3-20) than those of the psychiatrically
  trained interviewers. Trends in a similar
  direction and of a similar magnitude have also
  been described by Rodgers &amp; Mann (1986),
  Wing et al. (1977) and Cooper et al. (1977).


PSE is the former name of SCAN more or less the WHO-equivalent of the more commonly used SCID. Both are semi-structured interviews in which open-ended questions may be asked, but the SCID allows more freedom, including skips and completely novel questions from the interviewer.

With this long preamble, there has been a lot of research on instruments that would allow a layman to diagnose mental disorders with some level of reliability. These include CIS-R, CIDI, and even the older GHQ. These generally have a short[er] format and don't try to cover an entire diagnostic manual.

Trying to produce something that would beat the SCID (or even the SCAN) with lay interviewers didn't go well, insofar. Probably the most substantive effort was NIMH's DIS; the combination of its complexity (trying to assess the majority of  DSM-III diagnoses) and use of lay interviewers didn't go well. E.g.,


  Eaton et al., (2000) reported on 349 subjects reinterviewed by psychiatrists using the Schedules for Clinical Assessment in Neuropsychiatry (SCAN) following a DIS interview. Results indicated that the DIS missed many cases judged to meet criteria for diagnosis using the SCAN (29% sensitivity), and agreement between the lay versus clinician administered assessments for major depressive disorder was only fair (k=0.20).


(That's frankly understating it fair bit. A kappa of 0.2 for depression is pretty abysmal.)

Besides


  Due to its highly structured format, full administration of the DIS-IV typically
  requires between 90 and 150 minutes. To shorten administration time, the
  modular format makes it possible to drop evaluation of disorders that are not of
  interest in a particular study. Another option is to drop further questioning for a
  particular disorder once it is clear that the threshold number of symptoms needed
  for diagnosis will not be met. Although designed for use by nonclinician administrators,
  training for competent administration of the DIS is necessary. Trainees
  typically attend a 1-week training program at Washington University, during which they review the DIS manual, listen to didactic presentations about the
  structure and conventions of the DIS, view videotaped vignettes, complete workbook
  exercises, and conduct several practice interviews, followed by feedback
  and review. Additional supervised practice is also recommended.


So even this layman-oriented interview is not exactly an exercise where experience and training don't matter. Basically, the obvious conclusion is that the more fine-grained your diagnostic manual is in terms of disorder classification, the more experience and training is required to use it, even with fairly standardized instruments. (Of course that doesn't even bring into discussion whether all diagnostic categories can even be reliably distinguished, even in the best of circumstances, interviewer-wise.)

There's not a lot of research on the use of SCID by non-professionals because its use this way is discouraged. I did find one paper which did attempt this although the contrast was not with another SCID interview by a clinician, but rather with the consensus lifetime best estimate (CLBE) diagnosis (given by professionals) taken to be "gold standard". 


  The overall agreement between the SCID-derived [they mean SCID applied by a trained non-clinician] and CLBE [consensus lifetime best estimate] DSM-IV diagnoses was very low (kappa 0.18). The SCID tended to diagnose more patients with major depression and fewer with schizophrenia and schizotypal disorder. It was clear that the major source of the diagnostic differences was information variance. A brief, or sometimes monosyllabic response to a question of the structured interview was often short of crucial psychopathological information that might have been elicited in an epistemologically more adequate manner.
  
  Fennig et al showed a high diagnostic concordance for schizophrenia and bipolar disorder between the SCID diagnosis by non-clinician interviewers and the best consensus diagnostic estimates performed by psychiatrists. However, that particular study was limited to patients who were already diagnosed with a psychotic condition. More importantly, the SCID-elicited information also served as the main source of the best consensus estimate. This implies little information variance between psychiatrists and non-psychiatrists, i.e. clinician- and non-clinician raters made their diagnoses upon similar information. Minimizing information variance is exactly one of the cornerstones of the structured interview, but it does not guarantee the quality (validity) of the information.
  
  [...]
  
  In conclusion, this study demonstrated a poor diagnostic performance of a structured psychiatric interview conducted by a for-the-purpose trained non-clinician. Such an interview approach cannot be recommended for clinical work, due to a high proportion of misdiagnoses. The high specificity for the diagnosis of schizophrenia (100%) may suggest a limited applicability of such interviews in research with the purpose of a confirmation of an antecedent, clinician’s diagnosis.
  
  


What I also find interesting in this study, although not highlighted by its authors, is that the non-clinician categorized correctly all [CLBD] cases of major depression (using SCID). The [sub-]sample was rather small though (14). On other hand, he misdiagnosed a whole bunch of other [CLBD] cases (36=50-14) as being major depression. (High sensitivity, low specificity). That's not too surprising to me because even in primary care depression is generally overdiagnosed "for every 100 unselected cases seen in primary care, there are more false positives (n=15) than either missed (n=10) or identified cases (n=10)"; physician training as well as resorting to reassessments seems to lower this bias.

And that brings me to another comparison/point I wanted to make: if it's hard for primary care physicians to diagnose mental health problems accurately, it's not reasonable to expect laymen to do better (in similar circumstances).