Visually detecting a rear main seal leak?How?

I'm presuming this is engine oil.....If you have a full dust cover on the transmission, pull it off and see if the front side of the flywheel is oily. If it is, it's probably the main seal or gallery plug leak. If the flywheel is dry, It's something else.....oil pan gasket, intake gasket, oil pressure sender, valve cover gaskets, etc. You may have to clean off all the old oil to see where the leak is coming from.