Check your gas cap, fuel door or owner’s manual. Many flex-fuel vehicles have a distinct yellow gas cap, a sticker inside the fuel door, or a badge on the exterior that says “FFV” or “Flex Fuel.” Phillips 66® E85 is only intended for use in vehicles identified by the manufacturer as a flex-fuel vehicle.
You can also learn more here: https://www.fueleconomy.gov/feg/flextech.shtml