When someone is a PhD, and you know this, and you choose to use ms/mr/mrs instead of dr (or omitting an honorific altogether, the most common, unobjectionable choice), it can easily be interpreted as condescension.
Dude, PhDs give a fuck about PhDs. At least in the States. I would find it odd to be addresses this way. When I addressed my phd supervisor the first time with professor doctor XYZ, he just said, I am Bill. My name is Bill!
Also like the one time you most typically use the Dr honorific is specifically when you are speaking about the person in reference to their profession.
How do you know if someone is a PhD? They'll tell you.