Is there a telltale indicator that I need to change my cassette...
Not really. There are clues - have a look at it, see if a fresh chain slides much on the most worn cog, how many chains have you been through since it was changed etc.
...or should I just have expected a bit of cable stretch and rewire with a new chain?
Is your cable is stretched and your chain is worn?
If you're not sure what you're looking at talk to your fave LBS.
Could be
Worn chain, worn cassette, worn chainrings, old or dirty cables, bent mech hanger, damaged mech, old mech with a lazy spring or a combination of some of these
Not really. There are clues - have a look at it, see if a fresh chain slides much on the most worn cog, how many chains have you been through since it was changed etc.
Is your cable is stretched and your chain is worn?
If you're not sure what you're looking at talk to your fave LBS.