How to tell amount of propane in tanks ?

[grafton guns]

We purchased a tank level gauge that screws onto the tank then the hose screws into the gauge. I have two tanks marked 1 and 2 I use number one. When it gets close to empty I'll use number 2 BTW, I had the tanks full in 2016 when I purchased my 2017 Eagle HT 27.5 RLTS And, Have not had to use the second tank yet! I run the fridge on auto as I use the campground power. And only use the propane enroute to the campground to keep the fridge cold. I have a separate 20lb tank that I use for my Q2000 or the Blackstone griddle.