You can temporarily strap a voltmeter/multimeter to your triple trees and use alligator clips that are insulated to clip onto the terminals you want to monitor. You can use a SAE cable that most chargers utilize if they have disconnect cables to the battery and clip into that,insulating the probes with some electrical tape so nothing can short out. No welding with the battery or fireworks allowed...
This way you can monitor the voltage while riding. If you are going to have the bike stationary for much time you need to put a good box fan blowing over the exhaust and cylinders so it doesn't overheat the motor. Running a few minutes is fine,just if you are going to be idling and revving the motor for much longer than 5 min you would want to consider a fan...
Battery Tender makes a small voltmeter that plugs into the SAE charge cable. They also make a USB power port that plugs into it as well if you want an easy way to say charge or power an old phone or gps on the bike.
Two options for the voltage monitoring...
The one that plugs in and you press a button to check ot after it has been plugged in a while...
https://www.batterytender.com/Battery-Tender-LCD-Voltage-Indicator_2The other thing they have a Wifi enabled dongle that plugs in and you can access it through the app they have.
https://www.batterytender.com/battery-tender-wireless-battery-monitor_2While the second one sounds like a cool solution, it really is not imho because it has to be connected to a home wifi or other wifi to work if you set it up to do push notifications.
So, it is only useful when the bike or battery being monitored is in range of that wifi signal.
They should have just made it a Bluetooth and wifi capable system... Would be much more useful.
Lots of cheap small voltmeters available from ebay sellers, drawback is unless they are domestic sellers the postage wait can be a long time these days.
Remember, red letters can be hard to read in bright sunlight. The cheap ones typical are not waterproof, so keep this in mind. Easy enough to put it inside a plastic baggie...
Odds are it is going to be a temporary thing, so it need not be fancy.