There are a few different types of propane gauges
one is the "in-line" guage, it normally has a red/green dial, if it's green there is liquid in the tank,, RED = No liquid in the tank or.. No liquid in the SELECTED tank if you have an auto-change over, valve in front of your regulator
The next type is a tank mounted gauge.. This is usually a float, and how accurate it is.. I can't tell you.. floats in theory are dead on but in practice... Not always
Next is the magnetic liquid crystal thing.. (or adheasive) you slap it on, pour boiling water over it (Heat the water with propane from the tank, this improves performance), and the color change IS the level of liquid in the tank.. No question about it.. NOTE: my dad used to "Sound" the tank (Thunk it and note the level at which the sound changes. on the big 100lb bottles at home.. About the same accuracy.. Touch can be used too (The liquid spot is much cooler than the gas above it)
Next we have a fairly new/old entry in the field.. Scales
When I re-filled those old 100lb bottles at the sore.. They used to scale them. (Very old beam balance)
Today, there is a company that makes a stress-guage, scale.. This "Gauge" weighs the tank, if you know the tank weight (programed in when installed) and there is nothign that is affecting it (Like ahold down bracket) it can tell you exactly how many pounds of propane you have left.
But the tank mounted "Screw in" gauges I've seen were all float type. Kind of rare on bottles, but at least one company makes bottles with 'em.