Usually you can scan for specific ports.

I have found that if you do a basic nmap command such as nmap -sS ip it comes back as no computer being there. But if you do a nmap -sS -P <port number here> ip it usually comes back as say


port 80 HTTP Filtered

Thats one way I know Port 80 is usually almost there so it should be active unless the administrator is blocking traffic on port 80 and routing it to a different port.