Sometimes you can tell. I'm very glad I went to university for my undergraduate, and very glad I dropped out of my PhD. My career has been in fits and starts and I've been poor most of my life, but in my PhD I was doing a subject I wasn't interested in and I could see my career path ahead of me - disinterested, unmotivated post-doc, academic politicking with the sharks, no real-world experience with a doctorate in something that was rather tenuous and not useful, trying to hide this fact from future academic employment. Interestingly my first year in industry after dropping out gave me the experience that my topic would have ended in a null finding. The direction of the PhD was such that there weren't many doors opened by it, but once you have a doctorate you're overqualified for just about everything outside the doctorate's field - which sucks if you're not an entrepreneur (like me).

The important thing is to discern the people who say 'best thing I did' and actually mean it with the benefit of self-reflection versus the people who use it as a defence for copping out and not doing the hard yards.