How to find out whether iphone is original?

I'm buying a second hand iphone 4s and he says i can check it properly before buying so wht shuld i checkout in the phone to find out whether it is original or not and also where can i find the serial number of iphone and where can i type it to check it?

Check the IMEI or Serial No on the Apple website.

You can enter in the serial number here:

https://selfsolve.apple.com/...Dynamic.do

If the site returns the correct device type and model, then it's almost certainly genuine. It also will be genuine if it can connect to iTunes on your computer and/or the iTunes Store and download an app and also you can browse through this link http://support.apple.com/.../kb/ht3939