You can check the pressure with a gauge set. The low pressure port is near the battery. If you've got normal pressure, you might have a bad low pressure switch. I'd think it would be on one of the hard lines, but I'd have to look up the location.
To find out where the leak is, you fill the system back up and try and find it. Use R134a with dye added and grab a UV light and glasses/goggles.
Once you've found the leak, you can fix it. The strategy will depend on where it is. If it's a hard line, you can replace the line or just splice it. If it's something like the condenser, you'll need to have it patched or just replace it. You'll need to evacuate the system to work on it. Ideally you would use a vacuum pump and a recovery tank instead of venting R134a to the atmosphere, which is frowned upon.
Once it's fixed, charge the system again. Don't forget some PAG oil for the compressor.
As mikes is suggesting, you might let the shop have another look at it. From personal experience with this platform, after the compressor and accumulator have been replaced, the next suspect is the return line from the accumulator to the compressor. It's not available from Ford, but Vintage Parts should have one (Any good dealer parts department will look up the part and get it from Vintage). I think there is only one line that's not available from Vintage. I can't remember which one, but I remember that it's one that it should be possible to replace with an aftermarket / fabricated solution.
Keep in mind that the defroster setting engages the compressor. Continually spinning it up enough for the low pressure switch to get a reading and shut it down is probably not good for it. If you're not going to fix it for the winter, you should consider pulling the relays in the box under the hood to keep the compressor off. Having the system low/open for a long period of time is not good for it.