Why do you rely on credentials at all? With some thought and experimentation, you can develop a series of tasks and assignments that will require job candidates to demonstrate their capacity to read, competently use software and equipment, deal with customers, track down information using online search engines, etc.
One thing I have found useful in assessing general ability to absorb and retain significant blocks of subject matter is to review a candidate's transcript (and any who claim a diploma or degree, are required to provide a certified transcript as a prelude to scheduling an interview), pick out a few standard courses that I know in some depth, and quiz the candidate on the fundamentals of the material covered in such courses. If they don't have a clue, I know that either they never digested the material in the first place, or "pressed clear" when the course was over. In either case, my interest in them declines rapidly. I ask them to direct me to the parts of the transcript that they think they DID digest, and ask them questions about those courses. If they are clueless even there, - - - -bye bye (or, at least, credential value = 0). If they have a degree with a dissertaion, I ask for the title, check it out at the national repository for dissertations, ask for a copy of it, and then ask them background questions on the content.
If you think (with some reason) that credentials are an unreliable indicator of capacity to do the job, then you should develop your own measures of capacity, using help from others in your organization to probe areas with which you are not personally familiar. We generally have at least half a dozen people from different areas interact with the candidate in this way, and save ourselves a lot of grief in hiring by doing so.
>>If they don't have a clue, I know that either they never digested the material in the first place, or "pressed clear" when the course was over.<<
Depends on who you are interviewing. My undergrad degree was in Economics. If you ask me a macro question (even 1 month out of school), you'd get the "don't have a clue" response since I honestly didn't quite get macro. But if you wanted to talk micro and industrial organization, you would have found that I was quite well versed at the time.
However, now, I'm not sure I remember much of that stuff other than marginal cost = marginal revenue as the sweet spot on the curve (not even the best way or even a good way to phrase it). And I'd have to think long and hard to remember why. But I did grasp the overall broad concepts economics teaches and use those regularly.