Rear Main Seal?

I have a 428 with 4 speed that leaks alot at the bottom of the inspection cover. It appears that it is the rear main seal, I dont see any leak paths down the rear of the block from the intake, or from the valve covers. Is there a positive method to pinpoint that it is the rear main leaking? How feasible is it to replace the rear main seal with the engine in the car? Does having it attached to a 4 speed make it worse? It is a nuisance to get puddles under the car all the time.
Thanks,
Tom