I would think it would depend on the truck scale? You're in Napa, so does that mean wine country? I would be looking out for scales at those wineries, forklift scales I would think would be common, or at least not unheard of. Truck scales at an ag related may be down to 10-20 lbs increments vs the hwy scales that are 50.
That said, if you have an older dial type bathroom scale weigh the front wheel then weigh the rear wheel. The wight will be pretty close to accurate. I dug around and found this thread on ADVrider
and this video
skip forward to the 5 minute mark and see the comparison between weighing with 2 scales vs. 1. Interesting!