Take a length of rubber hose, like a fuel line, hold it to your ear and run the other end around where you think the leak might be. If it is much of a leak you can hear and feel it.
Bill
This is the easiest and quickest way, there's no doubt if there's even a small leak, use some clear vinyl hose about 3/16" about 3' long.
Originally Posted by Rdgray
You could always fart in the end of the muffler and have a buddy sniffing around to smell if he can locate the leak.