A good tip on telling which roadside vendors are sanitary or not is watching which ones have a lot of business. This is true of all places, since people flock to their favorites. You will be able to get a decent idea just by looking at a vendor. But also, most people blame these places for getting sick, when it is not their fault. Travel makes people sick, since every location has different bacteria, and your own biology is off from jet lag, and stress. People having problems is more from that than food poisoning, but we always blame the food.
I think just walking around is an important one. It is not the same to walk around landmarks and tourist sites. But getting an idea of the people, or normal life, just by going down the streets is important. It is the best way to observe how cultures are different, and the same.