Wow, how did she manage that?
SPOILER ALERT FOR THE ONE PERSON WHO HAS NEVER SEEN USUAL SUSPECTS.
Watching it again, there are 2 options. Either you believe Keyser Soze is real or you don't. If you think he is real, then it's likely to be one of the main characters and that's Keaton or Verbal.
The way the cop lays out the case for Keaton is too obvious, leaving Verbal as the only other option.
But she thinks while watching movies. I tend to just sit back and let them wash over me so the twist works great.