There are two ways to know how much is left in a canister. Get those Brunton stickers that go on the side of the can, or start weighing the canisters after each trip to track usage.

A 4 oz canister (the smallest one) weighs something like 7.14 oz. Empty it weighs (hey!) about 3 oz. Depending on the efficiency of the particular stove you have, which will be clear to you the first canister you use til empty, you can estimate how many boils you have left in the can. Which is not as hard as it sounds. Of course, the less gas in the can, the less efficient the stove gets, and that last half ounce or so just leads to the stove burning out silently without boiling.... But then you take it home, punch holes in it, pitch in the recycle bin, and get the next one.