What are the Thai tipping habits for meals? I expect that it depends on type of establishment: street stalls, budget restaurants, upscale restaurants. I've read that it's common to leave either 10% of the order or coins from the change: which one of them is correct, or are they applicable to different cases? How can I tell from observed features of establishment the amount of tips they expect from the customers?
Tipping is generally not customary in Thailand. For food specifically, there is no expectation 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 in line with what many others suggest.
Tipping is typically not expected in a Thai restaurant. But a lot of the chain restaurants will have a tip jar at the cashier. But occasionally if I am in a good mood, I will leave change or 10-20 baht. Maybe a bit closer to 10% at a sit down restaurant if I was impressed. For example, if it was like 280 baht for a meal at a sit-down restaurant, and you were satisfied with the service you might just leave the remainder in the checkbook or when the person is giving you change just hand it back to them immediately and say something like ให้ทิป (hi tip). Not too many venues that I've seen will include the service charge (10% or so); that is more common in Malaysia.
They should never ask you for one though. But it's not like Japan where they will actively refuse the tip. The bill is the minimum payment required...and anything over that is "up to you".
Tipping is a little more common (although not required) in service industries like beauty (barber/hairstyling, private taxi, massage.
As @Itai said, there is indeed no tipping in restaurants, or anywhere else, really. While a large chunk of Thailand is geared towards tourism, and fleecing tourists, tipping is not something that reached restaurants. While people in many tourism-related industries (taxis, massage, etc) expect, sometimes even demand, a tip, they can do this on a case-by-case basis. Asking for a tip in restaurants would mean that all customers, including Thais, would be submitted to it -- which wouldn't fly with the Thai customers...
[NB: I travel to Thailand every month for work]