is there a good way to find a vacuum leak in the evap system on a 1999 trans am?

with the motor running carefully spray either on the vacuum line when you hear a change in the idle you've found the leak.

Crude but effective - I rigged up a schrader valve from a bicycle inner tube to a piece of rubber tube and connected it to the vapor system. I pumped gently with a hand held bicycle pump (a few strokes, slowly and minding whether the pressure was building up) and could hear the hiss of escaping air. I followed the sound to a cracked hose. That method is not something you want to do carelessly. I'm sure it would be easy to pressurize the system to the point something breaks.

A more sophisticated method, which uses vacuum instead of pressure, is to connect a vacuum tester (see Source). The gas cap may allow air in so the gasoline can be displaced as you drive so you may have to duct tape over the cap. Anyway, you look for the leak by isolating parts of the system.

On the low-tech end you can connect a fairly long rubber hose, pressurize by blowing in it and kinking the tube to hold pressure. It's hard to build up pressure that will make a leak easy to hear, though.

Take it to a repair shop that performs smoke testing. A machine pumps an inert smoke (often with a UV dye) into the EVAP system, and then the tech looks for smoke or signs of dye, helping to pinpoint the source of the leak. Many shops will do this for under $100, it is safe for the evap system and usually quite reliable.