how do you 'know' if you've contaminated your beer? does it have a bad smell, skunky taste, etc???
Skunky taste from what I've heard. Never throw a batch out until you've tasted the final product.
Sometimes there will be a white ring in the neck of you bottles if you have an infection. Other than that, taste and smell. No pathogens can exist in beer, so it won't hurt you to try it. It'll taste awful though.