I buy fruits and vegetables from the market myself all the time, but with ready-made food - let's be honest - I always get nervous. I got food poisoning once from a pie at a market stall, and that's enough to make me pickier. Roskachestvo is right that the danger is real, but it's not really about the fact that it's "street food" - it's about how this business is actually organized.
The main thing I look at now is not just cleanliness around the vendor, like people mentioned above, but also the logic of storage. Mayo-based salads, raw meat, cottage cheese in hot weather with no thermos and ice pack? That's a red flag. I check if the food smells off, if it looks dried out or weathered. Another important thing: if the vendor is eating what they're selling themselves, that's a good sign - people rarely poison themselves. I ask when they made it, and if they give a vague answer, I don't buy.
One thing that helps me - I only buy what I saw being made. Pancakes they were cooking right in front of me two minutes ago? Yes. Dumplings that have been sitting in a container for who knows how many hours? No. If the food is hot and they made it while I was watching, the risk is much lower than if the vendor pulls something out of a thermos where it could've been sitting and going bad.