I second Gene's comment.
In addition, I use a small luggage scale to hand weigh tanks that feel light. I got one (similar to the one in the link below) at Walmart for about $6-7 in the luggage section.
I write the FULL weight on the top of the tank in pencil right after filling it. Then, when one feels light, I just weigh it to see how much propane remains. (The empty weight is stamped on the metal tank.)
Amazon.com: Luggage Scale - Hand Held - With Tape Measure By Travelon Bags: Home & Kitchen