DA,
I have known some very good psychics (and I have known some bad ones).
One is to watch how well they do over a span a time. No one psychic is 100% of the time, but the record will show which ones are good and which ones are just guessing.
Another thing is to watch their personality. Are they kind? Are they conniving? Is money their first motivation? This really goes hand in hand with being good at giving predictions.
See how good they are at telling you things about your past, your family, etc., that you already know.
Very important is seeing how pushy they are about money, and trying to get you to spend more money with them.
It takes a while, but good psychics can eventually be sorted out from the bad ones.
Being psychic is a skill, just like any other skill. It takes some talent and a lot of practice. Some people have a little psychic talent, some people have a fair amount, and some people are very skilled psychics. The trick here is to size a psychic up and see where they fit on this scale.