2000 Mustang 3.8 Base...rear Seal Leak Help?

There are no specific tests... you'll just have to look around. If the leak is dripping from between the bell and block, it's quite likely to be the rear main. Look at the sides and top of the bellhousing/block to make sure it isn't dripping from above from something like a valve cover or oil plug on the heads.
If the leak isn't excessive (like a quart in less than 1,000 miles), it might be worth waiting to replace the seal until you have another good excuse to pull the transmission... like clutch replacement.