Can I use a 99p sim to check if my iphone is unlocked?

I have an iphone 4s with THREE which still has a year left. I'm selling my iphone and using the contract sim in a normal phone (obviously with an adapter). I filled in an online form and they said they'll charge me £15.32.
Can i just buy a 99p sim card from an 02 store or something and put it in the iphone to see if it's unlocked?

Yes, your handset will not work and will ask for a code if your handset if locked.

They shouldn't be charging you in the 1st place. You've been on contract for a year. You still have a left. They should unlock it free.

If they as still insisting of charging you tell them your cancelling your contract. This scares them thinking their loosing customers.

If that don't work you can order a free sim from the net

You could use any SIM. That's not a problem.