I just repaired my home central AC by replacing the fan capacitor. Now I've cleaned the outside unit thoroughly, hosed off coils and vacuumed out all dirt from the fan/coil and compressor compartments, although I still have a few dents in the coil fins, most are fine. I even opened up the inside unit to see the cooling coil system (curiousity) and the vent system and all is clear and there appears to be no freon leaks. It's all sealed back up with high temp duct tape.

Now, how can I tell if the system is working correctly and efficiently? System is about 30 yrs old and I've never had it serviced. The freon line entering the compressor gets hot and the line exiting the compressor gets cold (not ice cold though). The inside vents blow cold air but not ice cold air. Is there a way to tell if it needs freon (because of a leak), even minor after 30 years? Should the compressor outlet line sweat from condensation, because it does not?

Bottom line, how do I determine whether I should call in a pro to check it out? I really like fixing things myself. What should a service check cost?

A service call may run from $60 to $100 depending on your area, you might check the temperature differential across the indoor coil, put a thermostat in the return air and then in the closest supply register, you should see about 20 degrees difference, if much less then this you should have it checked by an expert, more then 20 degrees means not enough air flow across the indoor coil.
The desire to repair things is why most of us got into this trade( not just a glutton for punishment). Good luck to you, Mike