You can get an IMEI checker for free on IMEIData.net and IMEI.info, as of September 2015. Additionally, iPhoneIMEI.info offers a service for specifically checking iPhone IMEIs.
Every phone has a unique international mobile equipment identity code, or IMEI. To identify your phone's IMEI, check whether it's printed on the phone's battery compartment. Alternatively, simply dial *06# on the dial pad.
The equipment identity register contains the IMEIs of all mobile devices that are connected to a GSM or a UMTS network. The database also holds other information regarding valid mobile phone equipment. When a mobile phone user reports his phone lost or stolen, the device's IMEI becomes blacklisted in the database. The phone then becomes unusable, even after a change of SIM card.
An IMEI check reveals a lot of information about a phone. Such information includes the brand and model of the phone, its manufacturer, its SIM card size, its phone band and its operating system. The check also shows whether the device has been blacklisted.
An IMEI code mainly consists of four groups. The first and second groups represent the phone's allocation body and the type allocation code, or TAC, respectively. The third group represents the device's serial number, and the fourth group denotes the Luhn checksum.