Samsung did make S2s in Korea, so that is what I could expect a real S2 to say. That doesn't prove it's real, but it's a good sign.
To really answer it we'd need more information about the phone, even pictures. For that it would be best if you could post in the main forum, where we could discuss it properly (since you can't reply to this).
What I've often found to be the best test is to look in the software information and check the baseband version. A lot of software information can be faked, but I've never seen that spoofed (because fake makers just have to use the firmware for the radio chipset they've included). So if you can look at that it can be checked to see whether it matches a real Samsung baseband.