Also, if it is one of the large canisters, crack the valve enough to let a bit of gas out.
I have an aquaintance who bought a "full" canister from a craigslist ad a few years ago. Weight of the full canister was correct, but when he got home and cracked the valve there was no vapor pressure!!
Turned out it was filled with water. He'd met the seller in a shopping center parking lot, paid $480 cash and had no way of accurately identifying the seller.
Gary