What is the difference between a crest reading volt meter and a peak reading volt meter?
Solution Summary:


The term Crest and Peak seem to be used interchangeably both by learned bodies [tm] such as ASTM and IEEE and by some Holiday Meter tester manufacturers. While it may seem obvious that "crest" and "peak" are synonyms, the discussions re waveform and risetimes make this not 100% certain. But ...
Some HM test manufacturers actually state that they perform Crest voltage testing.
If there is a difference it is subtle, but even if it is not solely a matter of terminology, it is clear from the literature available that use of a crest detecting meter will achieve your aims when used consistently. 




For the inquiring minds that want to know [tm].
 According to ASTM (who should know) 


A "holiday" is a small fault or pinhole  that permits current
drainage through protective coatings on steel pipe or polymeric
precoated corrugated steel pipe.
A "holiday detector" is a highly sensitive electrical device
designed to locate holidays such as pinholes, voids, and thin
spots in the coating, not easily seen by the naked eye. These are
used on the coatings of relatively high-electrical resistance
when such coatings are applied to the surface of materials of
low-electrical resistance, such as steel pipe.




This is based on what the net tells me:

This document Standard Test Methods for Holiday Detection in Pipeline Coatings
 - ASTM, Designation: G 62 – 87 (Reapproved 1998) provides a brief (3 page) but succinct description of the procedures to be carried out for holiday testing pipeline coatings. You can expect some procedural differences between pipes and general surfaces but the same principles apply.
 Also 1990 version

The document os old but what it says seems to be basically unchanged and knowing it exists should allow nwer versions to be located. 

The document uses the terms peak and crest 3 times, each, always together and always interchangeably. viz


6.3 Peak or Crest Reading Voltmeter—A kilovoltmeter
capable of detecting a single pulse and holding it long enough
for the meter circuits to indicate.
footnote 4: The sole source of supply of a suitable peak or crest reading voltmeter known to the committee at this time is Itt-Jenning ...
9.1 The instruments shall be standardized with respect to
voltage output in accordance with the manufacturer’s instructions, using a peak or crest reading voltmeter. This is used more
commonly with Method B where voltage may vary from test to
test but can also be used for veriﬁcation of the voltage on a
Method A test.




This SPY Jeepmeter Holiday Meter tester document says 


The SPY JeepMeter series of Crest voltmeters are instruments
designed to provide accurate and reliable measuring of the crest
value of high voltage non-sinusoidal waveforms. The output 
waveform of holiday detectors typically has this type of shape and
therefore prevents the use of conventional RMS voltmeters



This interesting IEEE document 
IEEE Standard Requirements for
Instrument Transformer uses the terms Crest and Peak interchangeably - sometimes in the same paragraph. eg page 74


a) Measure the crest open-circuit secondary voltage,  V1 [see part a) of Figure 31], using a 
high-impedance crest reading voltmeter, oscilloscope, or calibrated gap. Increase the primary 
current gradually from zero to the maximum continuous-current rating or until the crest voltage 
reaches 3500 V, whichever occurs first. Maintain the primary current for 1 min, and record the magnitude of the peak voltage. If 3500  V crest is not exceeded  by this test, then the information in item b) should be followed




Page 44 - 45 here is very relevant:
 EVALUATION  OF  CIVIL  WORKS METAL  STRUCTURES



Jeep Holiday meter tester

Several HMs

More

HM testers - Jeep

If there IS a difference, which is not certain given the above, it's not obvious what "crest meter" is meant to imply in the current context. Can you provide instrument brand and model. The term "crest" is notably absent from discussions of holiday meter testers by people such as jeep, who use the term "peak", but this does not mean that they are or aren't  synonymous. 

From a quick skim through the available internet material I'd say that consistency of test method is as or more important than instrument used. It should be easy enough to trial your crest meter in a range of situations and see if its reading seems to reflect your expectations.