You can measure toe and camber but not aster without alignment heads. (and slip plates are highly recommended)
I turn the wheel 20° left (driverside), zero the gauge, then turn the wheel straight + another 20° and thats the caster. I use my hub stands on car dollies (with jacking up the rear so ride height is the same) and then sometimes i will double check it with the tires on, on the ground, with the wheels sitting on garbage bags (cheapest turn plates out there). The same procedure that you get when you buy those camber/caster gauges. Adjusting it is easier on the hub stands as the tires not in the wayYou can measure toe and camber but not aster without alignment heads. (and slip plates are highly recommended)