All mobile phones have an IMEI number (International Mobile Equipment Identity). This number is part of an international system to uniquely identify each mobile phone. With the IMEI number phones can be identified and traced if they are stolen, or they can also be locked to a particular mobile operator’s network. The IMEI number can be obtained from the phone in order to check its number.
The website IMEI.info claims to be able to check the status of the IMEI number of each phone and to advise whether a particular phone is simlocked. If you don’t have another network sim card it may be the best way to check the simlock status for a phone.
I tried this with one of my old phones that I don’t use any more. It is only 8 months old but I got a new phone two months ago and soon I will be buying my second Android, so I won’t need that 8 month old as a backup. Along the way I discovered Vodafone now sells all of their phones locked out of the box. But if you can wait 9 months after you bought the phone, they don’t charge you anything to unlock it.