Originally Posted by Bryce
Pouring hot water down the side of the tank and feeling where the water turns from hot to cold always works for me.
+1, this is my preferred method. That, or weighing the tanks (a pain), or a built-in gauge are the only reliable methods.
The add-on pressure gauges are useless
due to how the propane is extracted from the tank. Propane gas is boiled off of the surface of the liquid propane, and that surface area is constant the whole time until you hit the lower "shoulder" (rounded part) at the bottom. At that point the pressure gauge will drop, but that is a very small time frame and you would be lucky to notice it.