Logs

Analyzing logs for attacks:

http://resources.infosecinstitute.com/log-analysis-web-attacks-beginners-guide/

Inspired by this talk:

https://www.youtube.com/watch?v=bDJb8WOJYdA

The successful attacker will know your network better than you do. They will know what is actually running on your network, as opposed to what you intended to run on your network.