The simplest check you can do is measure the voltage on the battery terminals when the engine is running. The voltage should be around 13.5 V @ 2500 rpm. If it's less, say 12.5 V or lower, then the alternator isn't charging. In which case the AMP-light should light...unless the bulb is broken...

A friend of mine had something different happen to his 66 Mustang, with the same result: dead battery. The same check as described above revealed a broken voltage regulator. In that case high revs would cause the alternator to overcharge (more than 16 V). 12 V batteries don't like to be charged much above 14.4 V.

I don't know how you hooked up the amp gauge, but check if you didn't wire it the wrong way around.
When the engine is not running, switch on the lights. It should lean to discharge. If it leans to charge, it's wired incorrectly. This check only works if the gauge is wired somewhere in between the battery and the light switch.

If it's wired in between the battery and the alternator (B+), then the test above doesn't work. The alternator is not supplying (nor using) any energy in that case (it's the battery). If the alternator is drawing current through the B+ terminal with an engine that's not running, though, then there's something wrong with the rectifier (the diodes) or the voltage regulator.