How can I verify whether my truck is charging my trailer's battery as I am driving?

How can I verify whether my truck is charging my trailer's battery as I am driving?

Hook up your truck and trailer in a towing mode. Test the voltage at your trailer battery with a multimeter and the truck not running. Then test it again with the truck running. You should get a higher voltage reading with the truck running.

Hook up your truck and trailer in a towing mode. Test the voltage at your trailer battery with a multimeter and the truck not running. Then test it again with the truck running. You should get a higher voltage reading with the truck running.
Another easier way to check is to look at your trailer gauges...If your battery shows a full charge when it is plugged in to your truck it is charging...If it shows 2/3rd's or lower you need to check your 40 amp fuse...