Test if your gas has two cycle oil added or not.

[Gadgeteer]

I just mark my variety of gasoline containers with the fuel/oil ratio when I fill them... the mark remembers, so I don't have to.

[Bony]

If I'm not sure whether oil has been added I simply test a sample on absorbent paper, well any clean white paper will do. Oil will leave a stain, petrol with no oil will evaporate away leaving virtually no evidence of saturation.

[Saltfever]

My gas/oil can is a different size (smaller) and shape from anything else. The smaller size assures that I refill sooner than the others with fresh gas/oil.