You'll know when you see scum in the headspace of the bottle neck after some conditioning time. Saying that though I have had infection that shows in the bottle neck but it was still great beer. Depends on the infection I guess.
Or fruit. That's what I got from infection I mentioned. (White sheet like patches).
It got a fruitiness character that I have read about and in reference to infection/off flavour but it was perceived as a complimentary flavour.
Fruity beer. It also served crystal clear and well carbonated.