The shop should have a gauge. If the shop gets testy when you tell them you want to check the fill, that's not a good sign. Make sure to tell them it's a HP tank the next time go you for fills and point the 3442 out to them on the tank itself.
Best way to get tanks filled with least trouble to you is drop them off on your lunch hour, if possible, and pick them up after work. Tanks get filled, and then topped off after they've cooled down.
Good advice, but unfortunately, I waited till the last minute to get the tanks filled for what might be diving Sunday. I'm fishing all day tomorrow and Saturday, so this was my only chance to get the tanks filled for the holiday weekend. Being in a hurry didn't help much in this case....