I don't always have access to the well caps, so most of that is out - sometimes the best I could do was to run hoses and reach steady-state pump operation, then observe the pressure gauge for falloff. The first time I got bit on one of these, the well was a solid producer for over an hour, and I had observed it operating for a good half-hour while I was estimating, but I wasn't paying close attention to the pressure gauge during the estimate, or I might have seen some fall-off. Most of these wells are sized for the house, and not for irrigation, so a few hoses will get them running steady.
I really like the air tube method. I wonder if there's a portable air rig for measuring water depth.