How to know if iphone 4s is unlocked?

I'm buying an iphone 4s off someone from a buying page on facebook who lives about 20 minutes away from me. I don't know this person, so how can it be proven the iphone is unlocked?

Be smart… Meet the person in a mall food court… Wip out the phone… Do not pay him, you require proof… Pop in you sim card… If you get a sim error, then its not unlocked… Refuse the sale, use the money to buy some food, and walk away…

If you get no error… And iphone allows you to make a simple call home/friend/pizza hut/etc etc etc (forget about text/data at this time, we'll deal with that later), and the call goes through without no issues, then its unlocked… Pay the dude…

To reply your question briefly. Ask for the IMEI number of the phone and check if it is unlocked and also check that it is not blacklisted. Such a check costs around a dollar or so. Buy here

Have a SIM from 2 carriers ready. If it accepts both, then it is unlocked.