Check out the NetServerGetInfo() function, there should be flags in there that will give you the info you need.

From MSDN:

Code:
#ifndef UNICODE
#define UNICODE
#endif

#include <stdio.h>
#include <windows.h> 
#include <lm.h>

int wmain(int argc, wchar_t *argv[])
{
   DWORD dwLevel = 101;
   LPSERVER_INFO_101 pBuf = NULL;
   NET_API_STATUS nStatus;
   LPTSTR pszServerName = NULL;

   if (argc > 2)
   {
      fwprintf(stderr, L"Usage: %s [\\\\ServerName]\n", argv[0]);
      exit(1);
   }
   // The server is not the default local computer.
   //
   if (argc == 2)
      pszServerName = argv[1];
   //
   // Call the NetServerGetInfo function, specifying level 101.
   //
   nStatus = NetServerGetInfo(pszServerName,
                              dwLevel,
                              (LPBYTE *)&pBuf);
   //
   // If the call succeeds,
   //
   if (nStatus == NERR_Success)
   {
      //
      // Check for the type of server.
      //
      if ((pBuf->sv101_type & SV_TYPE_DOMAIN_CTRL) ||
         (pBuf->sv101_type & SV_TYPE_DOMAIN_BAKCTRL) ||
         (pBuf->sv101_type & SV_TYPE_SERVER_NT))
         printf("This is a server\n");
      else
         printf("This is a workstation\n");
   }
   //
   // Otherwise, print the system error.
   //
   else
      fprintf(stderr, "A system error has occurred: %d\n", nStatus);
   //
   // Free the allocated memory.
   //
   if (pBuf != NULL)
      NetApiBufferFree(pBuf);

   return 0;
}