Very newbie question - just need quick answer

[Python49]

I'm sure this has been covered but a quick search didn't show results. I just bought a first generation 2g iphone unlocked. The iphone appears to not have a sim card in it and it is not set up with any carrier at the moment I don't believe (how could it?). How would I check to see if it's unlocked? Other threads said to pop in a SIM card, but if I don't have a SIM card in there at the moment, what would I even be looking for in order to know if it's unlocked? Would I have to pop in 2 different SIM cards from different carriers and see if both worked? Also, since I've never had an iphone before, what would it "working" even look like? I'd be able to make calls? Obviously very noob questions, thanks.

 

[r.j.s]

Put in a T-Mobile SIM. If it doesn't say SIM card error, it is unlocked.