If it’s labeled anything less than 100%, it’s probably got water or other compounds mixed into it. That’s how I assume it to be anyway. Methanol is methanol - nothing secret about it. It should contain no color and look like water albeit lighter. Methanol weighs .79g/mL, so a liter should weigh 790g minus the tare weight of the container it’s in. A gallon should weigh 2,986.2g minus the jug. If it weighs more than that by more than a couple percent, it’s got water or something else in it. More than 1% deviation and I wouldn’t use it for fuel. 2-4% water usually isn’t an issue, but that isn’t to say it won’t or can’t be.