Nitromethane is quite a bit more dense than methanol, if the containers are the same and pretty much the same amount of fuel is in each set them on a scale the heavier one will be the nitro fuel, or if you have a small scale put a small measure glass on there and pour a small amount of fuel in it and see which is heavier.
For the purposes of comparison of nitro vs methanol samples the lower ratio of water makes it easier for the OP to reach the answer. You are correct about contaminants and unfortunately it is not uncommon for commodity methanol to be less than pure.
Originally Posted by kschachn
Originally Posted by High Performance Lubricants
You can mix 30% water 70% methanol and shake it up. At that percent the alcohol will absorb the water and remain clear.
Methanol and water are miscible (all concentrations) and unlike what was mentioned before it will not be cloudy unless there is something else in the mixture that isn't soluble.