Well, if this is of great concern to you, there are some options:

(1) If you're a National renter, you can simply not select Flexfuel cars. If you are Hertz or Avis, you can presumably get them to switch you to a different car if you find one in your stall. Or maybe something in your profile saying "No Flexfuel cars."

(2) You can check E85 availability near the airport you're renting from. If there is E85 easily available nearby, then you can pretty much assume your E85 rental car will be full of it. If there is no E85 within 50 miles of your location, then you can reasonably assume it has regular fuel in it and you aren't getting ripped off if you refill it with regular fuel.

(3) If you're in a city with *some* E85 options, but not near by the airport, then you can do two refueling stops if you've driven most of a full tank. Fill it mostly with E85 and then top off with regular fuel. It's these cities where I'd probably try to avoid an E85 car...I wouldn't want to hassle with two refueling stops, but it would gnaw at me that I didn't know if the guy before me did. (My own airport is an example: we have a few E85 stations around the KC area, but the stations near MCI don't sell it that I'm aware of.)