There are only so many places that trans oil could get out.
Check the fill cap and drain plug. If these are loose, or have a missing or bad gasket (metal washer) they could be the source of the leak. Even a few drops of tranny oil creeping out under a loose cap will coat a large surface. It would be very nice if this were the problem because it would be easily fixed. Any of the other seals will probably require removing the trans to replace the seals. Or ignoring the problem and just adding oil frequently.
If the trans rear seal is leaking you will find oil where the prop shaft exits the trans housing. The front half of the shaft coupling will probably be wet with oil and flinging it all over the place (messy).
If the front seal is leaking profusely, you may find trans oil dripping out the bottom of the bell-housing. (It might take a serious leak before this shows up, there is a lot of room inside the bell housing where oil could hide. If the trans oil level keeps falling but no trans oil is escaping into the engine room, this may be where it is going.
Check seals where the shift controls enter the side of the trans, these might be leaking.
New oil leaking out will be easier to spot if all the surfaces are clean and dry. If you need to monitor for a while to find the leak, check the fluid level in the trans frequently. The trans holds very little oil so can't tolerate much leakage.
