Fundamentals - get to the race area early, sail part of a beat, see what's going on. At the very least, you can watch as others do this and see which side is favored.

Failing that, keep an eye on the competition like Matt suggests. Every once in a long while, leverage pays out; most of the time, though, if you're headed to a private corner and you don't know if it is favored, it probably isn't.