I probably should have been a little more specific. When I said "look at", I didn't mean simply "eyeball it". The bead (stringy thing) of caulk can detach from the substrate, but stay in place since it is elastic. The gap between the detached bead and the substrate can be very small and hard to spot. You can probe it gently with a small screwdriver, for example, to bring to light any bond failures.
The air conditioner is set on the roof on some sort of foam gasket, which is then compressed by screwing down the air conditioner. Occasionally this foam seal can fail, either because the airco has come a bit loose, or because it wasn't installed quite correctly. I won't try to tell you how to investigate this, because I have not done it. Other members have done so, and can help. If you have time for a deep search, you will find several discussions about this seal.
One other possible airco problem is that the condensate drain tube can get clogged, and the drain pan overflows. By any chance does your leak seem to start after running the airco for a couple hours on a hot humid day? If you can tell us the brand and model of your airco, we can hep you find the tube.
Good luck.
Bill
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