Most canisters have a tare weight on them and you can weigh the canister to see how much is left. However, most campers and outdoor enthusiasts will do the tried and tested shaking method to get a feel for how much is left in the canister. Some stoves use butane a lot faster than others so it’s always good to have a spare canister on hand when your current one is at about halfway. This gives you a good buffer so you’re not stuck without fuel for your stove.