Starling and her friend, Ardelia Mapp, have a conversation (after the scene where Lecter escapes from custody). Staling notes that Lecter said that "everything we need to catch him is right here in these pages". The subsequent dialogue goes like this:
Ardelia Mapp: Is this Lecter's handwriting? "Clarice, doesn't this random scattering of sites seem desperately random - like the elaborations of a bad liar? Ta, Hannibal Lecter."
Clarice Starling: "Desperately random." What does he mean?
Ardelia Mapp: Not random at all, maybe. Like there's some pattern here...?
Clarice Starling: But there is no pattern or the computers would've nailed it. They're even found in random order.
Ardelia Mapp: Random because of the one girl. The one he weighted down.
Clarice Starling: Oh, Fredrica Bimmel, from... Belvedere, Ohio. First girl taken, third body found. Why?
Ardelia Mapp: 'Cause she didn't drift. He weighted her down.
Clarice Starling: What did Lecter say about...?First principles"?
Ardelia Mapp: Simplicity...
Clarice Starling: What does this guy do, he "covets". How do we first start to covet?
Ardelia Mapp: "We covet what we see -"
Clarice Starling: " - every day."
Ardelia Mapp: Hot damn, Clarice.
Clarice Starling: He knew her.
This is how Starling knows that Bimmel is the key and that the killer is very likely a local. She proceeds to Belvedere, Ohio to Bimmel's home where she investigates the girl's room.
Another IMDb FAQ concisely outlines the ensuing events:
Following Lecter's advice -- that Bill covets that which he sees everyday -- Clarice drives to Belvedere, Ohio to talk with the parents and friends of the first victim, Frederika Bimmel. In looking over Frederika's bedroom, Clarice notices the the triangular gussets on Frederika's dresses match the pieces of skin Bill cut from one of his victims. Clarice calls Jack Crawford (Scott Glenn) to inform him that Bill is making a "woman suit" of real skin. Crawford is already in route to Chicago, having identified Bill as Jame Gumb, but they find that the house where he's supposedly living has been abandoned. Clarice continues to poke around in Belvedere and is eventually led to the house of "old Mrs Littman." Mrs Littman doesn't live there any more, but the current resident, who calls himself Jack Gordon, offers to give her Mrs Littman's son's business card and invites Clarice inside to wait. When she notices spools of thread and a Death's-Head moth, she realizes that Jack Gordon is really Jame "Buffalo Bill" Gumb. She pulls her gun on him, but Gumb leads her on a wild goose chase into the basement, where he has Catherine Martin imprisoned in a well. Gumb dons a night vision mask and turns out the lights, leaving Clarice to feel around in the dark. When he makes the mistake of cocking his gun, Clarice shoots him. Catherine is subsequently rescued. In the final scenes, Clarice is shown graduating from the Academy. At the party that follows, she receives a phone call from Lecter, calling from the airport on the island of Bimini in the Bahamas. He assures her that he's not about to come looking for her and requests that she extend him the same courtesy, which Clarice cannot do, of course. He then says that he must hang up because "I'm having an old friend for dinner," as he watches Dr. Frederick Chilton deplane.
While the characters in the movie might have been saying Mrs. Littman, the subtitles and the book both use Mrs. Lippman.
To summarise further, when Starling gets to the killer's house, she already knows a few things about him besides his psychological profile:
- He is a tailor/dressmaker.
- He has a thing for butterflies/moths and is known to have raised a Death's-head moth.
- He has a thing for "great big fat" people.
- He uses aliases using the same initials (J and G).
Each of these checkboxes are ticked during her meeting with the killer leading to her epiphany.