Ethanol Labeling Laws - State by State Guide.
Most states require gasoline containing ethanol to be labeled as such.
Ethanol-free gas stations in the U.S. and Canada
This site has a user-generated list of stations where ethanol-free gas is available. The site doesn't verify the information, so it could be wrong our out of date - but in most states, it will be labeled.
I'd encourage you to check the label on your gas pump, and add any ethanol-free stations that you know of to the list.
Too bad I just fueled up. I would like to try a tank of ethanol-free gas and see what happens. I guess I'll have to wait another 750 miles (fingers crossed!)