Possible Clues:

• 'Thick as a Brick' and 'Solid as a Rock,' e.g
• Rhetorical comparisons
• Many ''as''-in-middle phrases
• 'Smart as a whip' and 'sharp as a tack'
• They are literally not like metaphors!
• Phrases with 'as a' in the middle
• They use 'like' in a non-Valley Girl way
• "Mad as a hatter" et al