Too many people claim to have a Ph.D. When meeting a person who uses that title, you may ask them which university it was from. Is it a place like HKU, Oxford, Guelph, Yale or some other place that actually physically exists?
However be warned, some universities that have actual campuses are not recognised by their own governments as real universities. This recognition is called accreditation. There are many so-called universities that do not have government accreditation. So be careful when choosing a place to study.
One can also ask the person about the topic of their Ph.D. thesis. A real Ph.D. holder will be able to talk about the topic for days. A holder of a fake degree will often not be able to get beyond the depth of a Wikipedia page.
The title professor means a person who has the highest (academic) position in a department of a university1. Sometimes people in business or industry will have a position at a university, even if they are not employed there full time. They have a position at the university but a job elsewhere. If someone claims to be a professor, ask which university they are at. If they are not part of a university faculty, they are not a professor.
The title visiting professor is sometimes used as a courtesy title. Usually such a title is only for people who are already a professor at another reputable university. I was once invited to be a "Visiting Professor", when I was in Shanghai to give a talk for the Chinese Academy of Sciences. I was once listed as a "Research Professor" by an American university that (more importantly) allows me to use some of their resources. Again the title is a mere courtesy because of some things I have published. I have never called myself a professor. No honest person would ever claim to be a professor or doctor because of a temporary courtesy title.
If a person boasts of a fake qualification, be suspicious of anything else they say.