You guys have two very simple and effective methods on proving if your phone is unlocked or not
first (easy option)
1)if you have any family or friends that use a different carrier than yours and have SIM cards in their phones, just kindly ask to borrow it for a sec. and pop it in and see if you are able to make a call.
If that is not an option...
2)(takes a bit more effort but more effective) walk into a Popular (franchise) Mobile Carrier Store other than yours, simply ask the retailer that you signed up for a plan over the phone with them "the other day" and you are using this iphone with your plan "given to you as a gift from a friend". just say I want to be sure that this phone works with your SIM, before I get my SIM in the mail...Or to sound less conspicuous.... you can simply say you are shopping around for a phone plan, you do have a phone (your iPhone) but before you look into their phone plans, first you want to make sure if your iPhone works with their SIMS.
he/she should give you zero hassle and let you pop in one of their cards to test it, voila!!!
(if it's not unlocked, just say "thanks" and walk away...don't let him/her sucker you in to buying a new phone
hehe!
and if it is unlocked again...don't let him/her suck you in to buying a plan, just listen and nod and say "ok, thanks" and off you go)