Oil leak-what areas to look for 2001 Corolla

I got oil spotting on the tar on a 2001 Corolla with 135,000 miles on it. I am trying to locate the area it is coming from. Seems wetter on passenger side back of engine near top, and looks like it seeps and runs down to oil pan area and then drips on ground. I ruled out oil pan plug and checked oil fliter both look dry. Some seeping out of valve cover, but I am thinking there is also something leaking from back of engine. Any ideas appreciated. Slow leak so far, but need to repair.