Hi anchordriver,
I'm guessing from your question that your concern is as not so much how you'll know whether an engine has failed (all of the above are correct when they say "you'll know"!) as how you'll know whether the engine is damaged. And the reason for your concern, I am also guessing, is to do with knowing what to do once the failed engine is secure (i.e. after you've identified the failed engine, confirmed this with at least two parameters and then been through the appropriate recall items and checklists), because checklists, and we live or die by them, treat an engine failure differently from an engine fire, severe damage or separation in one important respect: do you attempt a restart or not? So, assuming it's not been on fire, you are looking for something that indicates whether the engine has been damaged (or dropped off!). I would suggest the following to start with:
-Engine vibration, felt or indicated
-Below minimum engine oil quantity
-Zero core or fan speed (especially if one but not the other)
-Visual confirmation from flightdeck or cabin
-Alerting System message implying damage
the Wings