Sight Down the Rear Derailleur Hanger

Stand behind the bike and look along the rear derailleur. The cage should hang vertical and parallel to the cogs; if it leans in toward the spokes, the hanger is probably bent. Hangers are designed to bend easily, so a crash or a fall on the drive side often shows up here first. A bent hanger can usually be realigned or replaced, unless it is built into the frame. The real question is what else took the hit. Check whether the derailleur itself is twisted, and look for scrapes or gouges on the bar ends and brake hoods, which suggest the bike has been crashed.