A volt meter across the battery terminals with and without the engine running will give you a general indication of the alternator condition. A good battery that is fully charged will read about 12.5V at rest. When you start the engine, there should be a voltage rise (nominally 1.5V) across the battery terminals. Hopefully you'll see something around 14V across the battery when the alternator is running. However, that 1.5V is nominal and varies from alternator to alternator.
Regarding the flickering light, that was much more common with generators than alternators. In general once an alternator "pulls in" and starts generating, the RPM have to get very low before it stops charging and the red light comes on. There are always exceptions though.
CraigZ asked about the Saturn conversion. (Hello by the way, nice to know someone local to me is on the board). I have a few acquaintances who have fit Saturn alternators to A-series engines but they have done the conversion themselves and not bought that kit. Delco is a possibility as are many Nippon-Denso alternators. Almost anything can be made to work if there is room and the pulley to mounting ear offset is close to right. I have a Delco on my GT6 and a Motorola (Bosch clone) on my Mini.