Switch on your mobile phone with a sim card from a network other than the network you are currently with. If you are able to use the phone, you can be sure that your phone is not locked. If your phone displays an error message saying "Wrong sim" or "Incorrect sim" or "Emergency calls only", etc., then your phone is probably locked. Otherwise your phone is unlocked.
If your phone is unlocked you can use any other gsm network sim cards. Or you should unlock your phone to use other network. Code is best way to unlock your phone. If you want unlock code you can get it from http://www.unlocking4u.com/ . They will help you to unlock your mobile.