Try getting a free sim from one of the other networks and seeing if it will work. O2 were giving them away on their website a while ago, Three usually do as well.
As was mentioned, if you purchased an iphone for a full price the only way to check is put a different sim card and see if it works. If it works, viola, you purchased an unlocked iphone.
That doesnt say or mean anything.
If you bought it full price and without a carrier commitment it should be unlocked. Did you get it from an Apple store or with wireless service from a carrier?
Why dont you test a few different providers sims to find out if it accepts and works with them.