I know that back when I did keto, I was checking my urine ketone levels twice daily for weeks before I was able to 'feel' if I was in ketosis. Maintaining ketosis was difficult and I had to closely monitor my food but I honestly found it to not be that bad a diet once I got used to it. Your milage may vary.
I guess I should just acknowledge that the article isn't actually addressing me personally, but I can't help but feel there are many people out there doing keto who are actually measuring their ketones and know whether or not they're in ketosis.
That's also the standard advice you'll get from almost anyone who gives advice on doing keto.