I've bought quite a few Thai posters over the years because they often have striking artwork, especially for genre movies. Often the Thai poster is better than the country-of-origin version! Also, for obscure movies the Thai poster may be the easiest to locate. There are Thai posters for Italian gialli and Hong Kong action movies that I have not seen much paper for elsewhere. In fact when I first got into posters over a decade ago, one of my first purchases was a roll of 20 or so Thai posters for Hong Kong movies for which I paid a ridiculously low price, and I have no doubt they are genuine.
I think of them sort of as the Mexican lobby cards of the Far East. They are cheap, cheerful and fun to collect, but are not really worth anything except in a few rare cases. Like Mexican lobby cards, they seem to have been produced in vast numbers and despite the harsh conditions of the country, a lot have survived (even though many times more must have been destroyed). This abundant access to mostly minor titles means that it makes no sense to make fakes except for the very occasional big-name title (the Apocalypse Now might be a case in point). Also, the printing process of the time means that many details will vary even from the exact same print run. It seems to me to be overkill to go the whole "poster forensic" route unless we are talking multi-hundreds of dollars material. And in that case, I would not so much go on what is printed but on the paper itself - does it feel and smell the right way for its supposed age? If this seems right the poster is most likely genuine. Even if there was a Thai poster forger out there somewhere, they definitely would not go the whole hog of printing on old paper!