Rear main or Trans seal leaking? 99 Ram

Truck: 1999 Ram 1500 5.2L, 5-speed (NV 3500?) 4x4. 118K miles.
I just bought the truck and crawled underneath and saw that there's oil on the tranny that appears to be coming from where the block and bell housing meet. My feeble mind says that it could be either the seal on the front of the trans or the rear main. Is there a way to ID what's wrong before I tear it apart? I'm planning on checking the oil level in the trans today. I haven't had the truck long enough to see if the engine oil level is dropping.
Thanks for any help you can provide.

ATF is red, engine oil isn't. Check the color of the fluid on the transmission dipstick, it might not be red anymore if it's burnt.

Engine oil leaks in that area could be a rear main seal or something on the rear of the motor like the pan gasket, OP sender, intake gasket, valve cover gasket or even the gallery plugs in the rear of the block.