How to display hidden characters by default (ZERO WIDTH SPACE ie. ​)
A very simple solution is to search your file(s) for non-ascii characters using a regular expression. This will nicely highlight all the spots where they are found with a border.
Search for [^\x00-\x7F] and check the box for Regex.
The result will look like this (in dark mode):
Not sure what you meant, but you can permanently turn showing whitespaces on and off in Settings -> Editor -> General -> Appearance -> Show whitespaces.
Also, you can set it for a current file only in View -> Active Editor -> Show WhiteSpaces.
Edit:
Had some free time since it looks like a popular issue, I had written a plugin to inspect the code for such abnormalities. It is called Zero Width Characters locator and you're welcome to give it a try.