If the compression isn't what it should be, you can sometimes tell if it is a leaky valve or bad rings by checking it once "dry," and then again after squirting some engine oil into the spark plug hole.

Theoretically, the oil will help bad rings seal better during the compression test, so if the compression goes up significantly, the indication is the rings are to blame.

If the reading doesn't change much or at all, you probably have a leaky valve(s) (or a bad head gasket).