How can you tell if your server is under DOS attack and that it has a virus, or have been hacked? Just random question.

Check the level of traffic to your server in orbit.
Scan your server using a virus scanner.
Check for root kits and which services are running - see if there is any strange ones.

Also checking through your logs is a very good place to start.

run "netstat -an" and look for a large number (like hundreds) of connections.