1) Look for the crowds. This may involve returning to a particular location at different times to verify its popularity. Taco trucks are most popular in the early morning, lunchtime and in the evening and late-night hours. But if you're rolling around the City of Angels and you see the glow of a taco truck parked along the street and there's four people chomping away on the sidewalk, that's probably a good sign.
2) For Mexican trucks -- ie, not those new trendy ones on Twitter (although they're good too) -- look for people speaking Spanish. If you can speak Spanish too, even just un poquito, try ordering in Spanish as well.
3) If you're trying to do an unscientific study of different trucks, and you want to know who has the best pastor, then make sure to do a controlled experiment. By that I mean, order the same thing (tacos de pastor) at each truck you go to. Then you can figure out which trucks you like.
4) Don't be shy about ordering something that seems odd. I think when most of us gringos roll up to a truck for the first time we start with something that's safe, like pollo or carne asada. But delve into chorizo, pastor and especially carnitas and you'll find a whole dimension of Mexican food that you didn't know existed. Beyond that, there's of course lingua, seso and cabeza -- I'm personally not a fan, but I'll try just about anything once.
5) Once you find your favorite truck, keep going back! Tacos are cheap enough that they're a delicious and affordable snack pretty much anytime. If you become a regular, and they start to notice you, not only will you feel good about supporting your local truck, but your taquero might start tossing you a few free horchatas your way.