And Then There Were None Topic Tracking: Clues
Clues 1: Everyone speaks in definite terms when discussing their invitations to the Island. Only Wargrave uses language that might have a double meaning: "Constance Culmington, he reflected to himself, was exactly the sort of woman who would buy an island and surround herself with mystery! Nodding his head in gentle approval of his logic..." Wargrave isn't thinking about satisfying anyone else's curiosity, he's thinking about how believable his story is, in case one of the other guests happens to know Constance.
Clues 2: Blore announces that there's a chance that there might be fingerprints on the letter, only Wargrave is alarmed by the possibility. No one else even reacts.
Clues 3: The voice refers reads out the charges in a formal fashion, and refers to all of the guests as 'Prisoners at the bar'. At this point, that piece of information isn't vital, but once it's clear that the murderer is one of the people on the Island, the question must be raised - which one of the guests would be most likely to set up their murders like a legal court?
Clues 4: With everyone else trying to discover who the killer is, Wargrave sets back their efforts by using his skills at summation to keep the guests from trusting or forming alliances with one another.
Clues 5: Despite his statements to the contrary, the first thing Wargrave does after setting everyone at each other's throats is form an alliance with the shakiest guest, Armstrong.
Clues 6: Wargrave suggests that they not bother searching for the revolver. He claims that there's no chance that it could be found, that the killer has had plenty of time to hide it. At every other opportunity, he's suggested investigating, but now he discourages it - why?
Clues 7: It is Maine's belief that only three people weren't guilty of the crimes they committed: Vera, Macarthur, and Wargrave. Vera and Macarthur confessed on the Island, though, and only Wargrave is innocent beyond a shadow of a doubt. It follows that the only person on the Island without a crime to be punished for must be the killer.