Oh oh oh I know this one.
Spray some starting fluid around the area you think is leaking. If the car stumbles and idles funny or revs up you have a vaccum leak.
The best bet would be to pull the top intake plenum off and check. That's what I just did and found it around the back side of the intake.
Also I heard like a pissing sound with the engine on. That's a pretty tall tale sign of a vaccum leak.