It's a given that Thai food is amongst the most delicious cuisines in the world so you only need to find the right place to eat! These two simple points should help:
1. Are you in Thailand? Many places around the world come close but it's never quite the same.
2. Is it packed full with locals with just enough space to squeeze in? If the place is packed full you can be sure it is great and this goes for small food stalls just as much as for nice restaurants! Thai people are passionate about food and eat out a lot so the good places will be very full; better stay away from the empty ones.
This is all you really need and you can be 98% certain before even having tried the food! chok dee krub!