Generally speaking, Thai food doesn’t have MSG in it. The oils, spices, and flavorings that are typical to Thai dishes do not usually contain MSG.
I can assure you that the Thai food here in Thailand that I eat almost every day doesn’t have MSG in it. It may have every spice but the kitchen sink in it, but there for sure isn’t any MSG.
The only way that MSG gets into Thai food is through various processed spices that may be added to it. For example, soy sauce often has MSG added to it, but soy sauce is not a common ingredient in Thai food.
Of course, the only way to be absolutely sure that your Thai food doesn’t have MSG in it is to cook it yourself, and even then it sometimes difficult to know for sure because food makers have gotten so skillful at labeling their products in ways that mask the presence of MSG.
Food companies know that consumers don’t want to eat MSG, so they label their products in such a way to try and skirt the issue. If you see “flavor enhancer” on the ingredients list, for example, it’s a good bet that it has MSG.
You can ask at your favorite Thai restaurant, but even upscale restaurants can be unaware of the presence of MSG if they are unable to accurately interpret the ingredients labels of the spices they add to their food.