I have doubts about some things, the first, how do you know when he is a professional player and when he is not?
If you are the fish at the table, you are not going to know it.
do they have to show some credentials?
LOL.
No.
I think that a person is a professional when they have done enough tournaments in poker and that they have been in the first place or in the first places, but here everything seems to be different, many call themselves professionals based on their own criteria, and that is something that can be tested.
What you think does not define reality. A poker player is a professional when he makes a living from it. Period. It doesn't matter what modality he plays. It doesn't have to be tournaments. Many play cash, sit and goes or spins, although everyone plays a tournament from time to time.