Your ears =P
Actually, you hand is the best tool. I would say wait till winter and look for steam, but that would be useless advice. When the car is cold, jack it up and start it. Then just feel around the flanges and joints. Big culprits are the header to h-pipe flanges, the crossover and where the pipes enter the mufflers.
I assume you have no cats to get clogged. You could 'fog' a cylinder or two. People sometimes pour a teaspoon of oil down their sparkplug holes when storing the car for the winter so it doesnt rust inside the engine. It's supposed to smoke like there's no tommorrow when you go to start her in the spring. You might fog 1 cylinder on each bank and then you'd have plenty enough smoke to test with! Might have to clean those 2 plugs though. Not sure how that would affect an O2 sensor though....If you had a carb, you could go WEEEEEEE and turn some screws and get black smoke 
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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