Diagnosing a slow fuel leak

Me and my housemate were working on my 74 Dart this evening, and he recommended I check for a fuel leak, based on the heavy smell of gasoline while the car was running.

The problem is I've never found puddles or other signs of a leak anywhere. I would not be surprised if there was a slow leak somewhere in the system.

Might you guys have any advice on trying to diagnose a leak?