I am having a problem with an oil leak at the back of the oil pan. Is there a good way to tell if it is the oil pan gasket or the rear main seal? If it is the rear seal, how hard is that to fix with the motor in the car?

Quik,
Jack it up high in the front and block it,take the flywheel cover off, clean the area with a spray brake cleaner. Buy a couple cans so you'll have plenty. Get a good flashlight and look under there with the engine running at a fast idle. It might take a while but eventually you'll see the oil coming out. After you've been under there a while you'll be able to make out the diff between the the crank, the main cap and the pan.

It's well known to be easier to pull the engine out to pull the pan off, don't even try it in the car.

Tom

Might try tightening the pan bolts first. See if it quits dripping after a few days.