For this reason, in the manual of every car, you will find the Octane number of the petrol grade your vehicle should use, that is the best grade for your vehicle, to perform optimally,
However, in the event you can't lay your hands in the car's manual, just check online for that vehicle's specifications, you will see the required Octane Number.
Oh... I think online would be the best way to go. As, I don't think most people have their manual considering the mode of purchase.