How do I know that removeEventListener was successful?

Is there a way to confirm that removeEventListener was successful?

Programmatically, as far as I know, it is impossible. The method does not return anything upon success, nor does it cause errors when a target isn’t found. See MDN Doc:

However, it is possible to debug in browser, and it so happens that your question was asked on StackOverflow a few years ago. Someone as a response described how you would debug the exact issue of removeEventListener() not being successful.