Pick it up and shake it. After doing it a 1000 times you can tell by the weight, etc. about whats left. Tare weight of most 20 lb tanks is around 18.58 lbs. (more or less) or printed on the tank + Tank capacity in lbs of LP -- 20 lbs?. More or less cause they don't always fill all the way up exactly right. So a full tank should weigh about 38-40 lbs. maybe.
At this point is is More or less, approxomately, Plum nelly, time to refill.
Use a 50 lb. fish scale for an exact estimate

Anything over 18 lbs or Tare weight is LP estimated exactly. (I carry a 50 lb fish scale in my back pocket -- always)
Scale -= weighing device, not what you scrape off the fish.
Words, words, words, in the English language are basically useless
If all else fails pour hot water over it -- I hate to boil water
Just wanted to make my valuable contribution to the discussion. What WERE we talking about

If it's not broken, you're not trying hard enough.