I use a luggage scale with a memory pointer feature. The memory pointer records the maximum weight encountered so I don't have to struggle with lifting and reading the scale at the same time.
The tare weight for my stock 30# cylinders is 26 lbs, so I prepared the following table which I laminated and is attached to each of my tanks:
Full - 56 lb
3/4 - 48 lb
2/3 - 46 lb
1/2 - 41 lb
1/3 - 36 lb
1/4 - 33 lb
Empty - 26 lb
Obviously, there is more work to this solution, as one has to remove the tank covers and the hoses to get a true weight.
I do this before my trip starts to get a baseline for where I am and periodically re-weigh during longer trips to determine a pattern and anticipate when there is a need to switch over and/or refill.
I realize that not everyone is willing or able to lift almost 60 lbs to utilize this method. My Weber grill at home has a simple usage indicator built into it where one hangs the tank from a built in scale with indicators at 0, 1/4, 1/2, 3/4 and full. Would love to incorporate something like that but would have to secure the tanks somehow for travel, something my Weber grill does not have to contend with.