I bought a refurbished, unactivated 4S. How do I check which firmware it has?

you need to get it running, in settings, general you will find your answer.

So I need to find a SIM from the original carrier to activate, is that all?

I bought a British phone, but I live in Belgium, will that work to activate? I thought there was an unlock and jailbreak for every 4S and it was so cheap, so I simply bought it without doing the research first.