Weigh an empty one. Weigh a full one. The difference is the amount of liquid propane in the cylinder. Weighing a partially full cylinder will give an indication of the amount left when you compare it to the weight of a full and an empty cylinder. Pressure in the cylinder will remain the same until all liquid is gone. Downs water method is a crude weigh scale.
MSR gives a little pictogram on the side of their fuel cans to give you an idea of what's left in it.