Look for the highest point where it's really wet. Generally there are a few places oil can come from, the valve cover, the head gasket, if it looks wet on the bottom but you don't see it very wet up top it could be the oil pan gasket, or the rear seal.
Another place could be the oil return tube that goes from the intake manifold to the top of the oil pan. The front main engine seal rarely leaks, I haven't had one leak on any of my cars yet.
Look for the highest point where the oil is wet or buildup is thickest and you'll discover where the leak is.
Performance one step at a time.