It's a pretty good indicator of where the pressure is coming from.

High volume on the bid means a lot of people are making bids to buy. The reverse is true when the volume is high on the ask. That means a lot of people are looking to sell. You only have to look at the first 5 or 6 volume #'s on each. Add them up and compare the sums. S'not a slam dunk but it's helpful.