Water is heavier than fuel. Pour the fuel in a clear container and let it stand for a day. Check it and see if you see a line where fuel stops and water starts. Because water is heavier it will be on the bottom. I use a cup with a rod in it to check the fuel in my real Cherokee 140. I insert the rod in the hole in the bottom of the tank and this releases fuel into the clear cup. If there is water I continue to do it until there isn't any sign of it anymore.

Gibbs