It could just be fluid comming out of the vent at the top of the pump? Make sure the trans is not overfilled. This is why it is common to plug the vent and re-locate it.
if not the vent issue, and it is in the converter bellhousing, you will have to remove the transmission from the car to fix it. I have seen a few torque converter hubs crack, and really leak alot of fluid.
About the only items I think you can check/fix with the trans installed would be the fluid level, the converter drain plug (if it has one), and maybe the plug for the front band lever pivot pin?
If it is not any of the issues above, then the only remaining seals in the bellhousing are the pump to converter seal, and the pump to case gasket/seal (and pump bolt washer/seals.)