If the scooter is equipped with batteries of a lithium chemistry rather than lead acid, the only reliable method to monitor state of charge is to charge it to a known capacity (in Ah or mAh) and use a device to record/display power used (in Ah or mAh). When you reach 80 percent used, charge immediately if you want your batteries to last any appreciable time.
Lead acid batteries can be monitored by voltage. A 12V battery should never be depleted below 10.5V and even doing so will reduce the lifespan of the battery. You can use the same time of meter/monitor for lead acid as for lithium chemistries, but don't deplete below fifty percent if you want a happy, long living battery.
In the hobby world, there are Watts-up meters. In the E-bike world, the CycleAnalyst is the top item. There are certainly going to be other devices that will work.
For all of the above, you'll have to open the case (voiding any warranty, as if that matters) to attach to the battery terminals and you'll have to be creative to provide a mount where you can see it while underway.
A typical installation involves a shunt, a conductive bar that connects to the battery terminal. The removed cable is then attached to the other side of the shunt. They are usually marked plus and minus to ensure that your energy use is not a negative value, but even that is trivial. There will be a sensing lead from the shunt to the display. Depending on the device you purchase, the meter will operate from the pack voltage (CycleAnalyst, for example) or you'll have to provide a separate source for the meter. Read the specifications carefully.