Not sure if Teflon looks any different when its worn out, but if it does its not much different. Exempting, of course, things like scratched & peeling Teflon.
If your pan is no longer giving you the nonstick performance you want, and you've tried basic stuff like giving it a good scrub (using a non-scratching sponge, of course), then it seems like you've answered your own question: if its worn to the point it no longer meets your requirements, then its at its end of (useful) life, at least for you.
(There may be ways to adjust what you're doing in the pan to get more life out of it; after all, a lot of us sauté our onions in stainless. But that'd be a different question, I suppose.)