Then if you're in the slightest doubt about a certificate, refer the whole thing back to the certifying body. Where's the problem? If you're going to outlay a significant sum on a certified stamp, why not go the extra step, and make a full check of the certificate? Stamps are far more individual than coins, after all, and a photo certificate will show the individuality of the item in question.
Incidentally, I know nothing at all of the situation with, say, US stamps, but I can say with certainty that I've never seen or heard of a forged certificate in the area I collect.
I'd still maintain that by far the greater risk is from certificates issued by bodies which aren't competent to certify the items. This crazy concoction

(Note the cancellation on the 1 Anna brown stamp at top)
came with a clear certificate from a certain individual in the US - no names, no pack drill - who seems to issue certificates like confetti, but obviously knew nothing at all about Barwani. And no: I didn't buy it on the basis of the certificate. It came in a mixed lot, and I effectively paid nothing for it.