This article discusses what goes on in the brain when hearing similes as opposed to metaphors. The article was created after interviewing 24 men and women.
Some metaphors are common in spoken language but are inappropriate in sign, body-part metaphors are possible in sign metaphors but odd/uncommon in spoken metaphor. In spoken language, tone and body position determine when a metaphor is that and not literal. Sign metaphors also have similar markers such as change in facial expression but are more limited in expressing change in tone and body position (as the body is already the main mode of communication). There is a sign for "metaphor" or a way to slightly change the signs for common words in metaphor so that the "listener" knows that the "speaker" means something metaphorically as opposed to literally.