I don't see what the label has to do with it. Isn't it the total amount that matters? If the externalities of a gallon of gas are $X, and the total taxes are > $X, then the purchaser is paying for them. How do you know one or more of the taxes doesn't identify as a carbon tax?

I guess you could assume that the current taxes are precisely applied to externalities other than CO2, but given the large differences in taxation due to jurisdiction, I don't see how they could broadly match up. And requirements for them to do so surely aren't strict or universal.