if he says that he is certified with ISTA, then he should be able to show you proof of his certification.
But I have another question: if his training methods are, by your observation, crude and he appears not to be very knowledgeable, why would you hire him even if he could show you proof of his certification.
I agree with Karen. you can also contact the ISTA, give them the trainers name and location (state) and they should be able to tell you if the trainer is certified by them. Here is their website
http://istacertified.org/ it does have their phone # too.