Pete,
Look for black oily wet spots, or the dye. Look at the condenser (in front of the radiator) for wet spots possibly caused by a stone hit, or a pin-hole leak due to corrosion of the core. Next look at the connections for the same thing, either dye or black oily wet dirt. Check all of the connections and look around the high and low pressure valves. Look behind the clutch coil pulley for a compressor shaft seal leak as well as at the seperation lines around the compressor heads for a gasket (O ring) leak, and at the manifold connections at the compressor. Lastly, look at the evaporator core for any signs of a leak. This is the hardest part of the A/C system to get a look at because of its location behind the glove box, etc. Also check the schrader valves for signs of a leak.
Yes, evaporator cores can leak. It is less likely than other parts of the A/C system, but they can and do occasionaly leak. The most likely place for a leak is; 1.the connections ("O"-rings), 2. then the compressor shaft seal, and 3. the condenser due to "pin-hole" leaks in the core.
