I'm think you'd almost have to buy from a breeder to be sure. Below is the standard for Ameraucanas-if you already have one that's supposed to be, I guess you could go down the list and see if it matches up:
AMERAUCANA VARIETIES (Large Fowl & Bantam)
Black, Blue, Blue Wheaten, Brown Red, Buff, Silver, Wheaten & White
AMERAUCANA SHAPE (Large Fowl & Bantam Males)
COMB: Pea.
BEAK: Curved.
FACE: Nearly hidden by muffs.
EYES: Expressive.
WATTLES: Small, preferably absent.
EARLOBES: Small.
MUFFS AND BEARD: Full, well-rounded, medium length.
HEAD: Medium size.
NECK: Well-arched.
HACKLE: Full.
BACK: Medium length.
SADDLE: Medium in length.
TAIL: Well spread, medium length, carried at 45º above horizontal.
WINGS: Fairly large, tips carried above the hock joint.
BREAST: Prominent.
BODY AND STEARN: Body of medium length, stern well-tucked up.
LEGS AND TOES: Medium length set apart, four toes
AMERAUCANA SHAPE (Large Fowl & Bantam Females)
COMB: Pea.
BEAK: Curved.
FACE: Nearly hidden by muffs.
EYES: Expressive.
WATTLES: Small, preferably absent.
EARLOBES: Small.
MUFFS AND BEARD: Full, well-rounded, medium length.
HEAD: Medium size.
NECK: Medium length.
HACKLE: Full.
BACK: Medium length.
CUSHION: Moderate.
TAIL: Medium length, well spread, carried at 40º angle above horizontal.
WINGS: Fairly prominent, carried above the hock joint.
BREAST: Full.
BODY AND STEARN: Full but trim, fluff well-tucked up.
LEGS AND TOES: Medium length, four toes.
