In general, on a modern scooter or motorcycle:
The alternator (stator) generates AC curent/voltage.
The rectifier converts the AC to DC
The regulator prevents the voltage from getting above 15 volts.
The battery stores the current until it is needed (engine off or current exceeds the capacity of the generating system).
So, in general, the battery should hold about 12.5 volts DC (no load or minimal load)
At about idle, the generating system may not make as much electricity as the engine and lighting system are using.
The generating system should generate about 14.5 volts DC at about 2500 rpm.
On many systems you can see the intensity of the headlight increase as you increase the rpm of the engine from idle.
Using a digital voltmeter on the DC mode, you can measure the voltage at the battery or electrical accessory outlet
and determine how healthy the generating system is as you vary the rpms. No need to exceed 2500 rpm on this check.
For Safety, you had better have the scooter on its center stand, rear wheel off the ground, and the front brake applied.
The best way to test a battery is to take it out of the scooter and have an auto parts store check it under load.
Also, if you have concerns about the battery, you should check the electrolyte level and add distilled water if the level is low.
Some new batteries do not have a need to check the electrolyte (AGM type batteries).
Scoot Safely,