Two types of tanks, different instructions.. First mounted tanks in most motor homes have a MECHANICAL gauge on the tank itself. This is fairly accurate, the remote indiactor inside is subject to all sorts of issues but the dial on the tank is usually good.
Also you can use the thunk method and the heat tape method (Actually the tape is an LCD crystal that is sensitive to temperature You apply the tape to the tank (it is either adheasive or magnatic) heat a pot of water on the stove using the Propane and dump it over the tape,, The color change will be clear in most cases.
Portable tanks can be scaled, Thunked or you can use the same LCD tape.