DDos Detection Articles:

https://www.hivelocity.net/kb/how-to-check-if-your-linux-server-is-under-ddos-attack/

http://linuxaria.com/howto/how-to-verify-ddos-attack-with-netstat-command-on-linux-terminal

https://support.plesk.com/hc/en-us/articles/360000345633-How-to-diagnose-possible-DoS-or-DDoS-attack-on-Apache