How do I find out what fuel my car takes?

The fuel type is usually printed or stickered just inside the fuel flap, and it's also stated on the V5C registration certificate and in the handbook. Petrol and diesel filler necks are also different sizes on modern cars specifically to make it harder to put the wrong fuel in.

If you've bought the car secondhand and none of that is available, the engine badge on the back of the car (things like "TDI", "CDTI" or "d" typically indicate diesel) can be a clue, but it's not something to rely on completely, since badges don't always match what's actually fitted.