Generally I would say:
3/4 time: You count in 3's, 1 2 3 | 1 2 3 | ....
6/8 time: The are 2 major beats: 1 2 3 4 5 6 | 1 2 3 4 5 6 |...
But if what you ask pertains to jazz interpretation, then stick with Mac's example.
Augmenting what Kevin said in 6/8 time beat #1 is the loudest, beat #4 is accented but not as loud as beat #1 and the others are not as loud as beat #4.
If beats # 1 and #4 are the same volume that the time signature could be either 3/4 or 6/8, at least that is what I have been taught.