How can you tell if a phone is unlocked?

[thelumberjack]

Got a Flipside that is supposed to be unlocked.  How can you tell if it is unlocked or not?

[FlipsideJewel]

Put your non-ATT SIM card in it and see if you can make a call.  If the phone isn't unlocked, you should get some sort of message about needing to have ATT service. 

[Remy]

Also, I believe that if you do what FlipsideJewel mentioned above, and your phone is not unlocked, it will ask you for an unlock code.

Thanks,

J