When your hands are really really greasy, dip your finger in the puddle. Now take the fluid and rub it together with another finger. If it creates a very clean spot on your hand, congrats...it is tranny fluid. Tailshaft seals do leak on occasion. Especially if you have had the driveshaft out recently and you hit the seal the wrong way with the yoke. If it is leaking from there, it should be slung up all over in the driveshaft tunnel and your transmission should start to feel a bit more notchy.