How to check if a Website uses Kirby CMS?
Are there unique things in the source code that are only present in the Kirby CMS?
Any idea?
I don’t think these things are necessarily unique to Kirby. You can make an educated guess if you look at page and image links and assets links, but I don’t think you can tell for sure.
There are means to harden your website and if you are in any way concerned, you should look into this.
The best way to secure your Kirby (or other website) is to make sure you use the latest PHP version, use secure passwords, make regular backups, secure your server, don’t use insecure FTP etc.