Connect the battery up and put lights on for 2 mins to stabilise it.

Turn the lights back off, take a reading with a multimeter set to DC across the two big battery terminals, expect 12.5volts ish, but it doesnt really matter what it is in this case, but you want it for reference.

Now fire the engine up, rise revs to over 3k to ensure the alternator is kicked into operation then allow to idle.

Now Take another reading from the same place and expect 13.5V +
What you are now reading is your alternators power output.

I suspect the reading will still be nominal battery votage, indicating your alternator output is zero.