This is the major problem with the GSM & CDMA Mobile Phones users, that how to check IMEI Number Information. There are the some guidelines to check or retrieve IMEI Number :-

On many devices, the IMEI number can be viewed or retrieved by entering *#06#

The IMEI number of a GSM device can be retrieved by sending the command AT+CGSN. For more information, refer to the 3GPP TS 27.007, Section 5.4 /2/ standards document.

Retrieving IMEI Number Information an older Sony or Sony Ericsson handset by entering these keys: Right * Left Left * Left *

IMEI Number Information on BlackBerry mobile phones can also be found by going to options, then status.