You can tell the front main seal because ATF will be originating from the torque converter cover. Gotta be careful telling that though because it will also get flung on everything behind it if you have driven the car on the highway.
Also, if you remove that cover and let the engine idle, it will make a huge fan of ATF on the ground from the converter flinging ATF off in all directions. Removing the converter cover could also result in lots of smoke because it will fling ATF on the hot exhaust pipe. I went through this after an engine rebuild on my Camaro. Freaked out because I thought I'd screwed up the rebuild!
