Visit any of the following types of stores:
Camping World/Big RV store
Hardware
Home Depot/Lowes/Mennards
Fireplace and grill store
Wal*Mart, or other "Super Store" (Grill dept).
They will have there a piece of tape, may be either magnetic or peal and stick, It's an LCD thing.. now we think of LCD (liquid Crystal Displays) as electronic devices but this one is not electronic at all. It is temperature sensitive.
You remove from package, Unroll and attach to the side of the gas bottle. You can cut it if there is a welded seam that causes issues.
now go inside and fire up ye old propane stove and put on a pan of water to boil.
When it boils.. Turn off burner go outside and pour the hot water over the strip
:You will see it change color.. at some point there will be a sharp or sudden change of color on the strip.. THAT IS YOUR PROPANE LEVEL.