I have a water separator, the problem is it isn't well lit, and I forget what color the diesel is. I swear it's different colors depending on where I get it. I only checked for a dividing line but didn't realize it had completely filled with water. It's happened before, too! I just kinda assume a filter with no line and a clear golden liquid = perfect operation.
@neftaly Yeah, I can back up your claim that diesel comes in a variety of colors. I've seen crystal clear to honey gold.
You could always drop a few drops of water soluable food coloring in. It will sit on the bottom and only mix water and turn it a color you won't be able to mistake.