I always have difficulty knowing when my Salbutamol inhaler is empty and was given a tip to help me establish how full the container was.
Simply pull the container off the holder and put it in a glass of water.
If it sinks to the bottom, then it's full.
If it floats, then it's totally empty.
Floating somewhere between the bottom and the top gives an indication of how much is actually remaining.
It works, and I find it very helpful to know where I'm up to as I'm getting older and more forgetful.
Well worth a try.