I have a oil leak on my 302 between the engine and c 4 auto transmission. What evidence would exist to show its a rear main seal or the oil pan gasket? 75 ford f100

 

Typicaly a rear main seal will leak into the bellhousing where a pan gasket leak is outside the bell housing.
Your oil leak could also be a leaking rear intake manifold gasket or valve cover gasket with the oil running down the back of the engine.
Best to clean everything thoroughly and do a daily inspection to see where the oil is coming from. Some shops have a dye they can put into the oil to help locate the leak,
Roger Carter