ἐξ ὀνύχων λέοντα τεκμαίρεσθαι → judge by the claws, judge by a slight but characteristic mark, small traits give the clue to the character of a person, deduce something from a small indication, identify a lion from its claws
εὔθρους, εὔθρουν και ἐΰθροος, -ον (Α)
αυτός που ηχεί δυνατά ή καλά («εὔθροα τύμπανα»).
[ΕΤΥΜΟΛ. < ευ + θρους].