Nope, this is as straight forward as it gets. There are only two ways not to know if you're failing or acing an interview:

1. You don't know what metrics you're being judged on.

2. You do know but you either can't assess your abilities in said metrics or you can't tell how well you've presented them.

The second one is up to the candidate, not much you can do about that, but if during an interview you have no idea what the interviewer is looking for, that's a terrible interview.