DC ammeter in the main circuit. Not moving is simple:
1. Engine off, nothing turned on, the ONLY thing on is the clock. That drain should be very small, as in a couple of miliamps.
2. Any significant amp reading, start pulling fuses to see which circuit it's in.
3. Identify the circuit then disconnect each item in that group until drain disappears.
4. Once offending item identified, either find out and correct its problem or replace it.
5. Verify unwanted amp drain is gone.
Engine running but not riding is similar to above.
Engine running but riding similar but you must stop each time to pull a fuse to identify the offending circuit. Even then you must avoid pulling the circuit fuse that is for either ignition, ECU or fuel infection.
You must have some voltage indication while riding because you said the voltage continued to drop while riding. I think that is a key that you are running "total loss" or strictly on battery. That puts the problem smack dab on connections among stator, regulator/rectifier and the system or malfunction of either stator or r/r.
