I can think of two basic ways, with a large number of variations on each of them. The first is to use somthing like nmap to analize the remote box and guess the OS. The second is to ask it. There's many variations on that, from looking at the telnet banner, through telneting in, to SSHing with the command-to-run being uname -a. I'd recommend either ssh/uname -a (or not -a, depending on exactly what you want, of course), or using nmap. Which depends on if you have ssh access to the box, and if your project spec and sense of elegance will let you run nmap. (I'd be very surprised if nmap is standard on prety much anything, with the possible exception of forensic kits, because it's often considered a "hackers tool" (and by "hacker", such people mean "cracker").)