There are many places on the outside of a trans that can leak. The 2 common areas are the front pump seal and the rear seal. Although they last a long time, they do spring a leak from time to time. The front pump seal is a bear to do because complete removal of the trans is required. The rear seal requires removing the driveshaft, but the transmission may stay in the car.
If you isolate the leak to one of these 2 area, simple replacement is not all you should do. Regarding the front pump seal, closely inspect the snout of the converter, where it passes through the seal. It should be very smooth with no nicks, burrs or heavy wear marks. The next thing to check is the pump bushing. It is easily visable after the front seal is removed. If it is chewed, discolored or ugly in any way, it should be replaced. Often a worn bushing is the cause of a leak. You do not want to do the job twice. A new seal will fail quickly if the bushing is shot.
The same principal applies to the rear seal. Driveshaft yoke must pass inspection. If not, replace it. The tail housing also has a bushing that supports the yoke. If worn, it should be change as well. Otherwise, seal will leak again, quickly. The rear bushing is a bit easier for the novice to inspect. It typically has small dimpls in the wear area that allow for a good visual. If some of the bushing has no dimples, or dimples that are less deep than other spots of the bushing, it will obviously be worn.
Bushing replacement requires special drivers and experience to install, It is best to have a pro handle this part of the job, if need be.

TTT