First, what is the length of the melody or chord changes? If they take 6 beats to repeat then it's 6 and not 3. Take Money by Pink Floyd - that's in 7, the movement of the figure in that song takes 7 beats to repeat.

To determine if a division is in 8th's or quarters is determined by pulse of the drums or tempo. Putting just a snare hit on 4 tells absolutely nothing. In fact for the most part I'd be willing to bet most drummers would play it very similarly to 4/4 except their fills would come in for the 5th and 6th beats instead of 3 and 4. If you're able to feel an 8th between the beat then it's likely in 6/4. If there is no in between on the beat it could very well be 6/8.

Most of it is semantic. All beats break down to 2's and 3's. So whether you count it as 6/8 or 6/4 doesn't matter, it's still counting to 6. Or it could be three groups of counting 2, or wo groups of counting 3. It's all in the accents and feel of how you're putting the beat together.

Jody Whitesides
A Funky Audio Lap Dance For Your Ears!
www.jodywhitesides.com