Visual clues will help most when trying to identify air conditioning refrigerant leaks.  Oil residue on any a/c component would warrant further investigation.  The photo below shows a typical condenser leak.  The addition of ultraviolet (U.V.) dye further enhanced the visibility of the leak.  Even without the use of a U.V. detection lamp the green dye mixed with oil was easy to see.

The leak in the example above was easy to spot.   The real trick is finding those tiny hidden leaks.  Hard core leak seekers know the secret to finding the smallest leaks. 

The Tip:

Read this article to find out what some pros do to find the smallest refrigerant leaks.