When the belts;
Can not be tightened anymore
Looked frayed or fuzzy, cracked and very hard when you try to compress with your fingernail
Seem tight but you get burning rubber smell when trying to mow and you get a noticeably lousy cut
Spindles/bearings, with belt off, see if there's more than just a 'little' play in them when you rock them back and forth. Spindles usually tell you when they need replacing, keep 'em greased.