1. The derailleur is horizontal rather than vertical when you're in top gear (because the chain has stretched about 6" in the last year).
2. You can't keep a smooth motion when on the small chain ring in back (because the teeth on that ring have worn down to the point where the chain is jumping).
3. The chain just isn't sitting right on the middle ring in front (because those teeth have worn into an interesting shark fin shape).
Anyone who wants to fill this list out to a top ten go ahead in the comments and I'll tell you if I've noticed those signs too. Oh, and did I mention that I've just ordered a new drive train?