If you give it gas at idle you'll hear a loud PSSSFFFFFTSHHHHHH
Thats a leak usually. However, on teh stock exhaust, with all test pipes, you'll get that same whistle, and its NOT a leak. I've had it with APS test pipes and Megan test pipes...i think its the 90degree bend in the stock Y pipe. You can also go under the car and have someone give it gas while you hold ur hand next to the flange. you'll feel some warm air shoot out or u'll smell it.

 

in the morning, turn on your car, look under, you'll see water dripping out from the leak also.