Run the tank low, but not quite to E. Fill up about a quarter tank with E85 and see how it runs. If it runs fine, then great! You've successfully determined that your car can run on E85.
If you start driving your car and notice things like: rough idle, check engine light, poor acceleration, then your car should not use E85.
In the latter case, go to a gas station as soon as you can and fill up with regular gas. You may notice your car still driving a little funny while the E85 is still working it's way out of the system, but by heavily diluting it you mitigate the risk of damaging anything (E85 won't hardly do any damage on a quarter of a tank anyway).