1) They should be verifiable meaning if they got their certification from a particular school or body, you should be able to check for that school online. This is the internet era. Nothing is hidden anymore.
2) When you ask questions, they should be able to answer lucidly and not evade your questions.
3) They should not be treating general dermatology cases.
A good soldier knows when to fold up and when to fight. Don't go treating what you're not supposed to treat before you get into trouble. That you mix cream does not make you an aesthetician or dermatologist.
As a patient...be vigilant. Be careful who you submit your skin to. Very easy to damage...but repairing that damage takes patience, time and it will cost you even more.