How can I check my website has not been changed?

I use a free Windows Store app, Website change detector.

It offers a simple point and click way to compare all the files in a local folder with those on my websites

https://www.microsoft.com/store/apps/9P7...

You can get all of the hashes (SHA256, MD5, etc.) of your files and then periodically check the files with the hashes

Or keep a backup of the files and then diff the server files with the local files