Honestly, I do not know how to answer this question. I cannot say that street food in Bangkok is 100% safe, but from my experience, I live in Bangkok and eat street food every day; I rarely have food poisoning. I mean once a year or once in 2 years that I get sick from street food.
In Bangkok, there are many food stalls on almost every street. It is very easy to grab something to eat in this big city. Besides, street food in Bangkok is delicious. Many times the food on street is more delicious than the food in luxury restaurants. The food such as Som Tam (Thai green papaya salad), grilled chicken, grilled pork on skewers with sticky rice, grilled fish balls are recommended to eat on streets because they are a lot cheaper and more delicious.
I have two tips for choosing street food, if you are afraid of getting sick from it. The first tip is choosing the popular stalls because you are likely to get fresh food (actually from my experience food from stalls on street are usually fresh) which is also delicious. It is not difficult to know which food stalls are popular. For the best stalls, generally you will see many people waiting in a long line to get the food. And, many of them bought a lot back home. Believe me Thai love eating street food so much. I have found some stalls that I have to call them in order to order the food an hour or two earlier before going to the stalls to get the food.
The second tip is always choosing the hot one, the one that is still on the charcoal grill. You can tell the vendors that you want that one and then wait. You might have to wait a little bit longer, but the hot one is not only safer, but also more delicious.
Have you ever eaten street food in Bangkok? How was your experience? Share it with me in the comment!