With a flex fuel engine, you can take advantage of high octane gas if you want - the knock sensors will allow more timing advance. You should get better power and mileage than you get on 87. Whether it's worth the extra 20 cents a gallon or not is debatable.

 

Mine is a 5.4L 06 and it's a FFV. The only identifiers are a sticker on the gas cap lid (cap itself is black with nothing written regarding ethanol) and a "V" in the 8th digit of the VIN.