i'm assuming you mean an ac condenser on a central ac and not on a vehicle since this is in the home and garden section?
If a homes central ac condenser is going bad you should notice corrosion and the aluminum fins corroding away exposing the copper or aluminum tubes on the condenser.
If the fins on the condenser are missing the ac won't cool the house down properly and then eventually the copper or aluminum tubes on the condenser could get little pinholes and leak the refrigerant out.