I never recommend "drive and deadhead at same time", since that requires looking and focusing in 2 different places at the same time, and the fact the chopper panel is controlling the drive function, and fly-back current is far stronger in the drive circuit. Just dead heading the hydraulics is far safer (especially to suggest for a non tech, or someone who might buy a battery forklift without being sure how to check the battery) and far more accurate.
Now if the unit has chopper hydraulics, then you need an amp meter and hydraulic pressure gauge to be sure how much load you are putting on the battery...