I disassembled my rear derailleur to clean it; when I removed the sprockets they basically looked the same; both jet black and caked with all sorts of black grease-like-stuff. I figured they were the same so didn't pay attention. However, after a bit of cleaning they turned out to be different.
One has a metal bushing, says Centeron G-Pulley, and seems a bit beefier. Which is the top/guide/jockey pulley, which is the bottom/idler?