I was taught, and pass on to students, to do an 'evaporation' test. Throw the sample on to dry concrete (NOT asphalt!) and watch. Fuel will rapidly evaporate whilst water will form globules with a higher surface tension. Holding a sample up to prove 'Bright, Clear and Blue' isn't always easy, especially if the sample tube has blue fittings on it. I found one such a while ago and threw it away. Yellow is my favourite for 100LL.
TOO