As far as ping, there is no way to tell. You might have to use a tool like nmap to determine what OS is running on said server. I am running a version of nmap for mac OS X that was written in Cocoa which I downloaded from sourceforge.net. I can scan the ports of any computer that I need to in order to indentify what machine it is and what ports are open on it.
I know that at work I often Telnet or SSH to a server (even if I don't have an account) just to see if its a Sun, AIX, Windows or Unix server. Around 50% of the servers I run into at work will respond with the OS before displaying the logon prompt.