How can I check my website has not been changed?
Is there a free app that can do this check?
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
In order to check the website's safety, I recommend making some research on this matter and use some tools. For example, Sitechecker offers such an option here https://sitechecker.pro/website-safety/ that checks the site’s health. Moreover, there is a lot of useful information about safety online.