You can check if the mobile phone you are using has previously been reported lost or stolen by running an IMEI carrier check on sites such as IMEIData.net and IMEI.info, as of October 2015. If the phone is an iPhone, run the IMEI check on iPhoneIMEI.info. Dial *#06# to get your phone's IMEI code. Alternatively, check is the 15- or 17-digit code is printed on the battery compartment.
The IMEI code is unique to every mobile phone. IMEIs of phones that are connected to a GSM or a UMTS network are registered in a special database referred to as the equipment identity register, or EIR. The EIR generally contains information on all valid mobile phone equipment. By conducting the IMEI search on any of the mentioned sites, you are simply checking whether your phone's IMEI has been blacklisted or marked invalid in the EIR. In most cases, a blacklisted phone is useless, even after changing is SIM card.
Apart from the blacklist status, the IMEI check reveals information about your phone, such as its brand, its model, its manufacturer and its SIM card size. If the phone isn't blacklisted, record its IMEI in a secure and easily accessible place as a precaution.