I think Peter.N is likely to have more idea than me, but I'll scratch the brain cell and give it a bash.....
The heavy duty lead goes straight to the battery, without a fuse. Diodes in the alternator prevent any current flowing the wrong way. I think this terminal could be marked B, or B+.
If you want to use an ammeter, it must be strong enough to handle all of the current the alternator can provide (70 amps or whatever). However, a 'shunt' ammeter can allow only a small amount of current through to register a charge. The larger current is allowed to bypass the meter.
The other terminal, sometimes blue, sometimes marked F may be used to operate a charge lamp. Its not as simple as that however, because the electricity to operate that actually comes from a switched live from the battery, to the charge bulb, and then to the alternator.....this could be used to provide an 'exciter' current for the alternator.
If you want to check if your battery is charging, use a voltmeter instead. These wire directly across the battery terminals and tell you what the voltage is. However, you don't want it to register all the time as it will eventually flatten the battery, so it is fed by a switched live, i.e it only operates when the engine is running.
DISCLAIMER!! Alternators are physically tough but electronically fragile. Reversing polarity of the battery or even allowing them to spin without the battery connected will goose them. So, be very careful if you fiddle.....it could be best to engage an auto sparky for an hour or two.
A simple check on your system.....connect your multimeter set to more than 12v across the battery. With engine off, see around 12v, maybe. On starting up, this voltage can decrease to around 10.5 but not much lower. Engine running, expect something around 14.5v.
If your battery loses charge over a couple of days for no apparent reason, disconnect the battery and see if it retains its charge.
John