Your log files are the best way to detect any trouble brewing. In your administrative duties, you will want to make it a part of your daily regimen. The key things to remember are:
What is the status code?
What is the user agent string?
What did the visitor do or attempt to do?
What errors did the system report?
If you see multiple attempts at something that is “just not right”, then block them.
See the .htaccess section for more.
Apache offers a number of error codes conveniently grouped into five areas. You will need to review your error log on a regular basis to make sure your system is working.
1xx Info / Informational
|101 Switching Protocols||HTTP_SWITCHING_PROTOCOLS ...|