Between the cop trying to convince Verbal he’s just a waste of skin and there’s no reason for him to protect Keyser and Verbal explaining his indirect dealings with Keyser, through Kobayashi, and how terrifying he is and that he wouldn’t give him up to the police even if he could, I think we’re absolutely led to believe that it’s someone other than The Usual Suspects.
IIRC, the cop does try to convince Verbal that it’s one of the other guys (checks wiki - Dean Keaton/Gabriel Byrne) but Verbal won’t believe it.
So, if you were to show the movie to someone that knows nothing about it and pause it when Verbal is released, I think most people would be going back and forth between whoever the cop said it was and whoever Verbal said it was and likely assume that the protagonist (Verbal, from out POV up until now) is correct, that Keyser is some faceless crime lord that they shouldn’t go messing with.
(I haven’t seen the movie in at least 10 or 15 years, so it’s also very possible (and very likely) I’ve misremembered some of the details here).