@Richman IIRC, it's all contextual. Some kanji only have one pronunciantion but some have two or more that change. I don't know any examples by head though, sorry.
@Richman For most words there are usually only a couple ways to read any given kanji and you just kind of learn which one for the given compound. The thing is though that proper names often don't follow the same rules. If you're talking about names the readings can be all over the place.