Originally Posted by Skyline

If you don't have access to a smoke machine, you might have some luck with more primitive tools. You could try an automotive stethoscope (9.99 at Autozone), possibly in combination with a vacuum pump with a gauge,($35 from Sears): Mityvac Mityvac MYMV8000 Vacuum Pump with Gauge - Tools - Mechanics & Auto Tools - Auto Body Repair Kits & Supplies

You might be able to hear a leak, and/or confirm that if the vacuum system is leaky or not.

Also, if you search the net, there are instructions floating around on how to home-build a quite inexpensive, automotive smoke machine.