How should I know when my rear derailleur is getting worn? I can measure/inspect chains, tyres and brakes; gears tend to show problems on a new chain if they're worn. But when changing my chain at the weekend I noticed the that there seemed to be quite a bit of play in the pivots - more than I'd expect from mechanical bits in general. Also the jockey wheels were quite worn (and didn't run very smoothly even after a good clean and dry PTFE).
In my specific case it's only Altus, with about 20 000 km on it, and the bike doesn't get cleaned as often as it should.