How can you tell the accomplice from the murderer when both claim to have been together?

Question

Accepted Answer

The accomplice really was in the room they named at the time of the murder, and the murderer was not. The accomplice never lies about the room, so the one whose room claim is false is the murderer.

Awkward Guests: The Walton Case

How can you tell the accomplice from the murderer when both claim to have been together?