In addition to the classic pre-departure checks, radiator fluid, oil level, windscreen wiper fluid, lights, remember to check how much gas you have in the cylinders .
If you don't have a specific measuring system, it doesn't matter, the best and cheapest thing is to weigh the cylinder you just bought and write the weight on it, and then weigh it again when you need it (a dynamometer for a few euros is fine), you will always have the 'information about how much gas you still have (there is no more precise method, it's just a little inconvenient because you have to remove and replace the cylinders to weigh them).