ἡ κέρκος τῇ ἀλώπεκι μαρτυρεῖ → you can tell a fox by its tail, small traits give the clue to the character of a person
failing this: P. and V. εἰ δὲ μή.
Best of all, let us make terms for ever. Failing that…: P. μάλιστα μὲν εἰς ἀΐδιον συμβῶμεν, εἰ δὲ μὴ… (Thuc. 4, 63).