A cloned N900 boots in portrait mode and has a grainy looking display with a black background. The back camera should have a dual-LED. There should only be a single SIM-Card slot behind the battery, and a number of copper colored circles (QC testing pads) should be visible. If any of these are out of place, walk away.

There's no difference between phones from different regions, except the firmware loaded on them. I'd prefer doing a reflash of everything myself, but it's certainly not necessary if it works fine. Bring a sim card to test, or otherwise call the phone.

Not much to say about good working order - make sure he hasn't done something questionable like attempting an overclock to 2Ghz. One possible result of dropping the phone is unseating the MMC, so check the free memory in the settings and make sure there's a 32 GB module inside.

Shake the phone to see if it sounds suspect (the MicroSD card holder makes a very light rattle when it's empty, but should otherwise not make a sound), test all the keys for stickiness, open and close the keypad a few times to see if it is still sturdy. Make sure all the side buttons and ports, especially the micro USB port, are all aligned and responsive. Make sure the stylus is there on the side and not chipped.

There aren't many accessories - you should have a USB -> micro USB cable, a wall outlet charging cable, and a TV-Out cable. Other accessories include earphones and cleaning cloth. I don't know about warranty service - I haven't tried to have mine serviced.