I check my propane......I check to make sure I have a spare tank so that I can change the tanks when it goes out. 

I know there are ways to check, pouring water on the tank to see the freeze line, a guage that I have been told is usually inaccurate and by weight.  I don't do any of those other than making sure I have a full tank somewhere.  Space and weight haven't been an issue so this method works for me.  I can see this being an issue if space and weight were a factor.

I use one of these its a pocket scale design specifically for propane it was 12.99 on amazon and folds up neatly. I keep it in a pouch just inside the camper door for easy access. it is exactly accurate.

you hook it on the top of the bottle and lift it up the gauge tells you how much is left in the bottle .