Filling a gas cylinder fully with water and then draining it will gas free it.
However, I do not think swilling an oil drum with water is a sure fire way of removing hydro-carbons. Last time I looked oil type stuff is not generally soluble in water!
If the drum had been used to store petrol or similar light spirits, you will probably find it is easier to gas free than something that has had diesel or fuel oil in, the light stuff will evaporate on its on accord. The heavier stuff may hang around and leave residue which produce vapour when heated.
Personally, if in doubt, I would always use a gas meter.