How to screen out candidates for faculty jobs who don't know the subject
Presumably your interview is doing a good job screening out the individuals who you feel "don't know the subject" and you are trying to screen them out prior to the interview. I think a reasonable screening tool could be a a phone interview. You should probably conduct between 10-15 phone interviews to find the 6 candidates you want to interview.

While I say "phone interview", it is most often now a "Skype" interview. These interviews could be as short as 30 minutes and should be no longer than 1 hour scheduled back-to-back such that they all get completed in two long days. The phone interviews I know about have had 3-6 people (presumably the majority of the committee) present, but I think depending on department politics that you could reduce it to a smaller group (maybe even just the committee chair).

While you could simply focus the phone interview on the questions that cause candidates the most trouble during on campus interviews, given 1/2 your candidates have difficulties recalling key concepts of "random" classes, you may want to help them out a little. If you require that applicants submit a sample syllabus for freshman physics and another for an upper level elective, then the phone interview should focus on the teaching statement and sample syllabus.

During the phone interview you should also mention some of the other key classes of you department that they might teach on. If a candidate is truly clueless, this will not matter, but good candidates will realize that they need to brush up on those classes for the on campus interview.

In summary, an inability to talk about a syllabus the candidate has written would be your screening tool and the hints during the phone interview will help you minimize throwing out potentially good candidates.