How do you determine a system is alive or dead in a network?

I tried ping, telnet, ftp and telneting to different ports, snmp ping. I also tried Nmap but it dose not work when firewall is installed (ex. XP with sp2).

Is there any other tools or methods through which I can determine the system status.
I can sniff as it is in network to determine the position, suppose if the system is outside network?

It will be very helpful.... Please let me know