Half-way through an average day, you'd expect the total volume traded for that day to be 50% of the average daily volume. 3/4 of the way through an average day (around 2:30-3:00) you'd expect to be at 75% average daily volume, and so on.
I have a chart that I periodically refer to for seeing how the volume is building for a stock I'm looking at. If it's running twice as fast (prorata) then I'm gonna sit up and take notice. If it's making a new high on normal volume or less then the new high is shaky at best, in my opinion.