Tipping is generally not customary in Thailand. For food specifically, there is no expectations to tip and most people either don't or round up the fare, leaving some of the change.
While I have been to plenty of nice sit-down restaurants in Thailand, I have read too that there are high end places where tips are more common and 10% is suggested but none of the places I've been to fit into that category.
So the basic rule is don't but if you are unsure due to finding yourself in a very swanky place, peek at what other customers leave. Beware though that those types of places may already add a service change, in which case no need to add your own.
This is based on my experience but is inline with what many others suggest.