if they look the part and act the part.: i look at their build and talk to them and ask for credentials of some sort. just figure out where they are coming from and their overall experience.
if they dont look the part, i ask for credentials and speak to their clients to see what they have accomplished.
...anyone who has watched branch warren or ronnie coleman train have seen their trainer (its the same guy) he doesnt look like much, but look what he can do to people. you might miss out on something if you straight up just judge by the overall look of a person.