How to know if my iPhone 4 is unlocked?

I took my iPhone 4 to be unlocked and when I went to pick it up they said they had done it but their system was down. So he said put a new sim card in and plug it into iTunes and it will say congratulations your iPhone is unlocked. I've done this and no message came up. My phone keeps saying invalid sim.

I've moved over to australia now and people of here say I need to get in contact with o2 so they will unlock my phone for free (which i didn't think was right) Other people here say you can't unlock your phone at all. Can anyone tell me how to see if my iPhone is unlocked and what I should do on iTunes?
I'm so confused with it all!

Easy. Just use another sim and if it didn't work, it means your phone is still lock to the network. You can call your former network personals which i think is o2 to unlock it for you. However, you can save yourself stress by just purchasing a cheap official iphone unlock code direct from iTunes. Try this website
http://www.iphone4sunlockcode.com/genuine-apple-iphone-5-unlock-code/

Only the original carrier can unlock the iphone 4 and 4s… Any site that sez they can is a scam.