Visually detecting a rear main seal leak?How?

If you see it below the level of the bellhousing, but don't see any ont he top, it could be any of the oil plugs on the rear of the block, or the pan, or the eal if it's engine oil. With a stick, make sure it's not transmission oil (which would be either ATF or gear oil depending on the builder/owner) out the input or input bearing retainer.