A quick way to do a basic test on an alternator is to get out your volt meter and check the voltage with the car running, and the car off. Running, it should be about 13.5V, even if you have lights, radio, etc on. If not 13.5V, your alternator is very likely to be the problem, but if it is 13.5V there is still a possibility that it's not putting out the AMPS, and should be bench tested.
Engine off, you should be right around 12V. This will give you a starting point. If the alternator is no good, go yell at the shop. If the regulator is no good then go get yourself a new alternator from your favorite parts store from a late 80's Chevy that is internally regulated, wire it up, and call it a day. (There's probably other Makes that work, but I use the Chevy alternators because the mounting holes seem to line up more often than not with any cars that I have retrofitted).