Here's a good one for when you're out driving looking for spots: look for shopping centers and the like that are built where there's a difference in elevation from one side to the other. Often, those are where you can find crazy loading docks, retaining wall ledges, etc.
I guess Google Earth (not Google Maps) shows differences in elevation pretty clearly now, so this can be done without driving I suppose. Also, if you live in a big city, the shittier parts of town usually have good stuff that isn't stopped in shopping centers and office/industrial parks, so either grab some homies to drive around for an afternoon looking for stuff, or do a Google Earth before you head out.