You can see it in the attached picture at the back of the trunk. This one has some custom pockets on the cover, but you get the idea. Even if the spare wheel isn't there, you might have a bolt sticking out that it normally secures to.
Good idea to check the pressure of the spare regularly too. I did when I bought mine and found it was at 50%. That would have been great fun if I'd needed it.
Edit: and another pic for good measure:
