To prove that a message really came from an owner of a Bitcoin address, get the person who claims to own the address to sign the message and send you the message with its signature. You can then use MultiBit to verify that the message was indeed signed with the address's private key. Whoever knows its private key effectively owns an address, so you will have proven that the message really came from an owner.
From the Tools select "Verify message"
The "Clear All" button will erase the contents of all fields.
If you have copied a Bitcoin signed message format message you can use the "Paste All" button to fill in all the fields instantly. Simply copy the complete contents of the message below and use "Paste All" to try it.
-----BEGIN BITCOIN SIGNED MESSAGE----- Hello World -----BEGIN BITCOIN SIGNATURE----- Version: MultiBit HD (0.1) Comment: https://multibit.org Address: 16R2kAxaUNM4xj6ykKbxEugpJdYyJzTP13 H0b22gIQIfutUzm7Z9qchdfhUtaO52alhNPK3emrkGOfbOzGHVPuWD9rMIphxniwBNgF/YN4c5C/dMwXz3yJz5k= -----END BITCOIN SIGNATURE-----