Can one claim that a person is having cognitive distortion and be always correct? What is cognitive distorion?
According to Wikipedia, &quot;cognitive distortion are thoughts that cause individuals to perceive reality inaccurately&quot;. It is a negative outlook on reality, sometimes called negative schemas (or schemata), and can reinforce negative emotions and thoughts.

Criticisms on cognitive distortion

Common criticisms of the diagnosis of cognitive distortion relate to epistemology and the theoretical basis. The implicit assumption behind the diagnosis is that the therapist is infallible and that only the world view of the therapist is correct. If the perceptions of the patient differ from those of the therapist, it may not be because of intellectual malfunctions but because the patient has different experiences.

My question
For convenience, I'll name the person who is making diagnosis Cueball, and the person who is being diagnosed Megan. Here is Cueball and Megan:

I agree with the idea that Cueball may even have assumption that he is correct (thus having a distorted perception about Megan), but I don't think this criticism paints a full picture. I think the main issue with cognitive distortion is that Megan lacks self-skepticism. That is, she doesn't assume that she can be wrong. Any person can (or will) assume that they are correct, but they can still assume that they can be wrong at the same time. And more importantly, the behavior of a person assuming they can be wrong is totally different from the behavior of a person who doesn't have that assumption.
So if Cueball doesn't see that behavior in Megan, can he conclude that she has cognitive distortion and still be correct? Is the signs of cognitive distortion so detectable that one can use gut instinct to conclude and still be correct?

Related: Does following logic necessarily require one to conclude that they are objective and have no bias?
The use of assumptions in clinical settings seems to play into the errors you mention. Cueball is just as prone to distortions as Megan.

According to Dawes, Faust, &amp; Meehl (1989), a professional in the field has to rely on one of two choices when making a decision on a client, either their own clinical judgment which is based on experience and knowledge or actuarial judgment which essentially rests on the statistical method, establishing a link between empirical data and the event in question. The clinical judgment does not seem to be accurate, for the most part. Clinicians fair better when using methods that are backed by actuarial methods, rather than relying on their own "hunches" or "gut feelings". 

It would probably be a better idea for Cueball to at least try to jot down some explicit instances of distortions and compare them to some baseline to ensure it is something out of the ordinary, at least for what he might know about Megan. 
Though he would be correct, at least in this instance, but it wasn't necessarily due to experience, but chance.
 Which is one of the big issues pointed to in the article by Dawes and his colleagues, they essentially find that the criticisms towards the therapist being an all-knowing being are valid and point out that they often forget to consider other variables that might be contributing to the situation, so with the occasional correct guess, they may begin to rely on their gut or worldview which distorts their decisions, overall. 

To combat these errors Lambert and Shimokawa (2011), highly recommended that client feedback is collected because it helps in understanding the client's subjective point of view and can aid in aligning the therapist's views to better meet the needs of their client, leading to stronger outcomes. The practice of feedback helps build a factor in therapy success, which is the therapeutic alliance (the relationship between client and therapist). Flexibility also helps as well.

This notion of client feedback is another layer on top of the common factors pointed out by Rosenzweig (2002) that seem to contribute most to positive client outcomes, more so than the theoretical method used. These factors include the qualities of the therapist and how they interact with the client and their specific qualities.   

Essentially, it is the nonspecific elements, like therapist and client qualities in interaction, that create the foundation for a better client outcome. 

Dawes, R. M., Faust, D., &amp; Meehl, P. E. (1989). Clinical versus actuarial judgment. Science, 243(4899), 1668-1674.

Lambert, M. J., &amp; Shimokawa, K. (2011). Collecting client feedback. Psychotherapy, 48(1), 72–79.

Rosenzweig, S. (2002). Some implicit common factors in diverse methods of psychotherapy. Journal of Psychotherapy Integration, 12(1), 5–9. doi:10.1037/1053-0479.12.1.5