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
cestros: i, f., = κέστρος,
I the plant betony: Betonica officinalis, Linn.; Plin. 25, 8, 46, § 84.