How to judge a potential graduate student based on a short interview?
When interviewing candidates for industry jobs, I concentrated on two issues:

Is the resume accurate, or if inflated how much? A resume is a sales document, and candidates do try to present themselves as favorably as possible, but it should be fundamentally truthful.

Can the candidate discuss technical issues in the relevant field?


Everything else is covered by the paperwork.
Fortunately, there is one form of question that can help resolve both issues. Pick a topic, such a research project they claim to have done or a course they did well on, and ask about it. Do they know as much about the topic as the resume indicates, and can they discuss it?
As Buffy suggests in a comment, zero error rate may not be possible. Ultimately, you just have to do your best and accept you will make mistakes.