What are the signs of an abused engine?

I've heard something about it not idling correctly....but what would be some tell-tale signs of an engine that has been driven hard?

Is there any way to tell without performing compression/leak down/etc?

 

do a compression test to check your piston rings and open it up and check your bearings. a bad idle is usually associated with emissions, sensor, vaccum or fuel issues.
there's no clear cut way to tell if an engine has been abused; to many variables!
it's a process of elimination.