You can run some hot water on the side of the tank. Then feel the tank where it stays cold. That's the level of propane.
You can also remove the tank and weigh it. The lighter it is, the less full it is. While you have it off, you can also swish it around and if you hear fluid swishing around, it has some gas in it.
How I run my tanks is I have an auto change over regulator (it's a 2002 model, so most any new rig should have this feature too) and I open both tanks fully open. There is a lever that you turn to point to either tank. The tank it's pointing to will be the ONLY tank you draw from.
You should also note that there will be a GREEN or RED band on top of the regulator. This indicates that the tank you are drawing from still has some gas in it.. If it's RED, that means you are drawing from the other tank and you need to swap out the empty one.
Move the lever to the other tank, remove the empty one, refill it, put it back on and wait for the other tank to empty. Repeat...
But, that's just what I do.. Does not work for everyone, so just gotta figure out what your propane usage is and how often you are willing to check the actual tanks and go from there..
Good luck!
Mitch
2013 F150 XLT 4x4 SuperCab Max Tow Egoboost 3.73 gears #7700 GVWR #1920 payload. 2019 Rockwood Mini Lite 2511S.