The other thing is that all major gasoline companies trade fuel with each other. Let's say that there is no nearby Chevron refinery, Chevron will trade fuel with another company that has a refinery in that area. That way they don't have to truck the fuel long distances. The only thing that is different is the additive package, which is mixed with the fuel when it is put into the tanker truck for delivery to the gas station.
I judge it by how busy the gas station looks/how nice or rundown it looks. If the gas station looks deserted and you never see anyone filling up, you have a higher chance of getting water in your gas as it sits in the underground containers. It has only happened to me once and that was in my car...not a fun experience. Quik-Trip has a guarantee on there gas, so I use them when I can. I have noticed a difference between 87 and 89 and run 89 about 90% of the time.