The color will change and become more yellow. Also, if you shine a flashline into the fuel, you may see bubbles. That's water. You used to be able to tell by the smell (rancid if bad), but I think that's changed with the ethanol blends.
Found this: http://blog.gasbuddy.com/posts/Maint...12027-322.aspx
and http://www.fuel-testers.com/expirati...hanol_gas.html