Just drain, siphon or pump some of it out, put it in a glass jar or something clear and see if it separates after a while. That will show the water content.
Vented tank of unknown age equals moisture issues probably. Boats are the worst, not so bad if the tank is full for one winter. But years of sitting in a vented metal tank will kill that fuel's usefulness and rust the fuel tank.
I just tried burning some E10 that's been sitting for over 6 months says the previous owner. It didn't go well. New purchase, who knows how old the fuel really is. I thought it was clean looking normal smelling fuel too, it ran okay for a couple times and then things went south. Hopefully I can just drain and refill with something a bit fresher...