How can I find out if a mobile phone is stolen?

try this http://www.checkmend.com/uk/

INFORMATION ABOUT BARRED MOBILE PHONES
Q: What is a barred , blocked phone?
A: A stolen or lost mobile phone and useless.
Q: How does a mobile get barred or blocked?
A: When a person buys a Pay As You Go phone, or gets one free, or cheap on a contract or pre-pay tariff on any network and the person either:
1. loses their mobile phone and reports this to their service or network provider.
2. has their mobile stolen and reports this to their service or network provider.
Q: What happens when a mobile phone is reported lost or stolen to their network?
A: The network provider then puts the serial number (the IMEI number for the phone) on a shared blacklist database. This list stops any phone on it from registering on any network. The phone is now no good to anyone; it should not be bought for spare parts and it is a criminal offence to handle it. If you find such a phone it should be handed in to the police so that it can be returned to the owner as he or she may have some important numbers, text messages or pictures in the memory of the mobile phone.
Q: How do you find out if a mobile is barred or blocked?
A: Common symptoms of blocked stolen or lost mobile phones are:
When a SIM is inserted in the phone it will seem as though the mobile is working and will show that you have reception on the network.
However if you try to dial out although it will seem that the call is going through, you will hear a number unrecognized tone.
If you were to dial the number of the SIM card you entered, the mobile would not ring when called.
The phone will again show reception but when an outbound call is made it will repeatedly fail. You will only be able to send text messages.
Note: Sometimes a phone will work on the Vodafone network for calls going out but the SIM will be barred after a while.
If you check it on other networks you can find out if the phone is barred as it will display the symptoms described for the other networks.
On O2 and Orange the SIM will say simcard registration failure, will have no reception and no network operator logo will appear.

Q: What should I do if I buy a second- hand mobile?
A: Check to find out where the second- hand or new phone you are buying has come from and whether the phone is on a contract or Pay As You Go. It's common for people to abuse the insurance on their phone by selling it, then later on reporting it as stolen. That way they get a replacement phone whilst the person they have sold the phone to loses out as their phone will be barred and blocked.
Make sure that if you buy a phone you register the phone with your network provider as if the phone gets barred at a later date you are mostly covered. Unfortunately there isn't a database that allows the public to call up an automated service and check if the phone has been barred and where the phone originated from.
Q: Is changing the IMEI the same as unlocking?
A: Mobile networks often lock handsets so that they are only able to operate on specific networks. The unlocking of handsets to allow them to operate on any network is not against the law and is not the same as changing the IMEI.

NOTE: this is a cut and paste from MY OWN web site and therefore does breach any copyright