Many canisters have the total weight printed on them in ink. Depending on how old the canister is, the total weight will be between 22kg and 26kg.
Some canisters have a metal chip in the outlet on which the total weight minus 20kg is indicated: so, for instance, “2.7” means 22.7kg.
If neither of the above is present, the empty weight is stamped into the base ring. Depending on how old the canister is, the total weight will be somewhere between 12kg and 16kg.