First and foremost is local patronage. The number one indicator of good street food is the prescence of a long line of hungry locals. Many a time I've foregone the busier places out of self consciousness and regretted it. People tend to be pretty helpful, and I've usually found someone just within my preiphery vision to help me out when I get stuck.
I also tend to favour one dish operations over multi-food extravaganzas. If a stall is selling paejon, soondae, dukbokki and tempura, chances are that one (or all) of them will be below par. Time and again my favourite stalls have been those that specialise in one particular food item. These places often combine passion, expertise and attention to detail in a way that shows in the food.
There are of course exceptions, but I've found that the best street food stalls generally don't tout for business. If they look like they don't need nor want your money, you're probably on to a good thing. You're also less likely to get ripped off.
It seems obvious, but I try to avoid stack-em high joints in favour of somewhere that will cook to order. This is especially true of the Korean Tempura and Pajeon stalls, where sogginess tends to set in after a relatively short time. Even grandma's old socks taste good as long as they're straight off the hotplate!
Also, avoid vendors outside tourist attractions. In my experience, these places consistently turn out below average food at above average prices. They also tend to fall foul of at least one of the points above, meaning they should be doubly avoided.
Finally, the most important aspect in choosing somewhere to eat is to follow your gut. Never feel pressurized into ordering something until you properly suss it out. A place might satisfy all the positive criteria listed above but if it doesn't feel right, don't do it!