Interviews are just a second rate way of evaluating someone for a job, even well thought out interviews combined with tests arent that great. And there are so many sites where you can find sample interview questions and answers that it really is moot.

The answer is formal professional standards, combined with formal mentored training/internships along the same lines as the medical profession. Then when you see that an applicant has reached the standard you know for sure that they have reached a certain level of competence. This will never happen in IT though because it would dramatically raise the price of IT employees. So I guess we will just have to make do