I bought a iPhone 4 off someone, how can I check if it is stolen?

I bought it off a random person that I know.

They said that it is with orange, I spoke to orange and they said the phone is definitely locked to orange. I tried the sim cards they sent me and none of them work and they can't understand why they aren't working. They asked me to test the sim in someone elses phone but I don't know anyone who has a orange network phone. They said they will call me back on friday (assuming I found someone with a orange phone.)

Is there another way to tell if the phone is stolen? Should I take it to the nearest phone shop and ask them if they can check? And explain the situation to them?

I will appreciate any responses.

You can either call your cellphone company and give them your ESN so they can run it in the system to see if it comes up as stolen or you can bring in into an Orange store or Apple store and have them run the ESN to see if it's stolen.