how to know emei number when phone simlocked

Hello I bought a second hand Iphone 4 simlocked on ebay, I want to un-simlock it, but the EMEI number on the box is different from the EMEI number from the iphone simcard slider! Apparently the guy on ebay said it was locked by UK orange, but without the correct EMEI number, how can I do that!? So I have been looking everywhere but I have not seen a proper way to check the EMEI number from the iphone Itself! And there is no EMEI number on the back of the phone either!