It depends on the frequencies we're talking about. Assuming we're talking about audio frequencies, one method is to square up the signals (through a schmitt trigger) and feed the signals into the capture inputs of a 8051 cpu with a PCA timer. We can measure the period of each of the signals and the time difference between them. Assuming both signals have the same frequency, the time difference between them is the phase.

If you can be more specific with your requirements - a more specific answer can be given.