If you want to check for water in fuel, put a small sample in a clean (and dry!) glass container and put a lid on it. Place the jar in the fridge to cool it and if there's any water then as it cools all the oil will suddenly come out of solution and fall to the bottom of the jar. The temperature when the oil falls out depends on how much water is in the fuel. As a guide, with zero nitro and a synthetic oil (the fuel I was testing) it'll seperate at freezing point (32F or 0C) when there's .75% water. I actually flew with 3% water added (on a warmish day) with no problems at all. YMMV