Street food in Southeast Asia

I just returned a month ago from spending 6 weeks between Beijing and Singapore with many great meals from street carts.
As mentioned, stand back and watch - which carts are busy and which carts are clean. In a way you actually see the kitchen. (I recall walking out one restaurant in China ... we walked past the kitchen, I looked inside and left immediately.)
The only problem we had was eating items with more "heat" than my tender tummy is used to .
We travel with sani-wipes and use them regularly
Enjoy.

It depends on the country. For what it is worth, in Northern Vietnam the only time I got sick was when I ate at a very fancy tourist-only restaurant. Absolute misery with a high fever...I figure the problem was that the restaurant did not have a high turn-over rate. (It was the crab spring roll, I am pretty sure) The pho stands on the street used only fresh ingredients and sold everything in a few hours. I never had trouble at any of the places frequented by Vietnamese people.
Obviously, don't eat raw veggies in India or Nepal, but you should make a case by case decision for other countries.

My spouse went to Thailand, Lao and Cambodia and ate from many carts and only got sick once from fried chicken. My friend just got back from thailand and she only ate from carts, she did not get sick but stuck as much as possible to a vegetarian/pescatarian menu.

Like Midge, I also follow the 140F rule and buy only bottled drinks (tea excluded)

I wrote a story on this years ago and one piece of advice --from an infectious disease expert -- that stuck in my head was that you can eat pretty much anything that's above 140 F, so that steaming
cauldron of pho is likely OK. Naomi Duguid (Hot Sour Salty Sweet, etc.) also pointed out that eating on the street actually makes more sense than in a hotel or restaurant because the vendors are cooking right in front of you.

I often look for the locals. Like Tony S wrote, you don't stay in business if you make your local clientele sick.

Avoid raw fruit and veggies... It is the water they use to wash these foods our systems cannot handle, not the fact that the food is rotten or infested with bacteria or some other pathogen.

As long as it is cooked, you should be fine. Remember, tourists are a small percentage of their business and they would not have many repeat local customers if they were making people sick.