Perhaps you just read the post on "how to know a locked and an unlocked phone" and now you'd want to know your iphone 6, 5, 4, 3 is an unlocked phone. then feel free because you are in the right spot.
I will like to tell you that the only way to find out is to get a sim from another network (if you currently use MTN sim, Get an Etisala or Airtel sim), insert into the phone and see if you can make a call with it. i know you may not have another sim card,if that is so, then get the sim of your friends (what are friends for?)... you must ensure the sim you're borrowing is the same kind used on your phone (if your phone uses a micro sim, then the one you're borrowing must be a micro sim). for example, iphone 4s and iphone 4 are like a twin when it comes to sim cards because they both use micro sim, also, iphone 5s and iphone 5 are a twin because they both use nano sim, so it will be best if you looked for a friend that uses the same kind of phone as you.
Now that you have got the sim card, insert into the phone and try to make a call with it. if the call connects, then its an unlocked phone, meaning you have absolutely nothing to worry about but if the reverse is the case, then you will need to jail break or call your network providers to assist you in unlocking the phone.
Hope you enjoyed this?