I'm no expert but from what I've read Araucanas should be rumpless (ie no tails) if you are in the USA whereas Ameracaunas should have tails, so pretty easy to tell them apart. Just to confuse you though, here in the UK and possibly elsewhere in the world, Araucanas have tails but then I don't think we have Ameracaunas. Anyway, the biggest confusion/deception seems to be between Ameracaunas and EEs(Easter Eggers) (sometimes labelled Americaunas to deliberately confuse or deceive) EEs and Americaunas are mixed breed birds that have at least one blue egg laying gene in their parentage and therefore may (or may not) lay blue/green eggs..... usually they do. Note one letter change from an a to an i makes the difference between a pure breed Ameracauna and a mixed breed Americauna (EE). Telling these two apart can come down to colour and is perhaps not so easy.
That's my take on it from across the pond, but I could be mistaken.
Regards
Barbara