Turn it on and use it
Turn it on and use it. Try doing the following:
Open a TextEdit document and start typing. Are all the keys working? Are any of the keys sticky or feel really loose?
Try and get on the Internet with it via Ethernet. Or plug in a phone cable and see if you get a dial tone in Internet Connect.
Look at the screen, is it blue-tinted or "normal"? Try using the contrast/brightness controls.
Does it make any funny noises (grinding, repeatedly clicking)?
Try opening a lot of software all at once and switch between them to give the RAM and HD a workout, does it Kernel Panic?
Plug in some USB/Firewire devices, does it Kernel Panic?
If you are getting it from an individual or from a local store do this BEFORE you actually pay for it. If it's being delivered look at the shipping box before you sign, if it appears at all damaged (or cheaply packaged) tell the driver and ask to file a damage claim and/or refuse to sign for it.