I don't suggest trying to check the battery while traveling, that has a tendency to have someone look away from the direction of travel, just long enough to eventually cause a real serious problem, and lifting to a "stall" puts more of a load on the battery than traveling.
I also use a 10% rule as to voltage drop, which, as we see more and more higher voltage batteries in the USA, will make a difference in the troubleshooting. if it drops 10% from the "at rest" voltage, then fixing the battery problem is the needed first step.