For a quick and dirty -- and assuming that this is a 30 pound cylinder -- do this:
1. Check the collar of the cylinder. You'll find "TW" followed by a number. That number is the tare (empty) weight of your cylinder.
2. Add the TW to 30. This will yield the weight of the cylinder when it is full of propane.
3. Put the cylinder onto your bathroom scale. It should read pretty close to the full weight established above, assuming that your bathroom scale is relatively accurate.
If the cylinder weighs more than what you'd expect, then it could be that your filler incorrectly relied on the OPD valve to fill the cylinder, and the OPD is bad. (Because OPDs are never inspected again after initial installation, fillers are not supposed to rely on them as a means to decide when to stop filling a cylinder.)
:)
Lynn