À l'ongle on connaît le lion; by just looking at his nail one can recognize a "lion." The sense is that even a detail is sufficient to recognize a great man; somewhat like the English expression, "you can tell a leopard by its spots."
un lion: a lion; figuratively a courageous, bold and daring man, or celebrated person
un ongle: a nail (as fingernail)
connaître: to know, in the sense of being familiar with a person or thing
une licorne: a unicorn
The above photo is a detail of one of the six tapestries of the Dame à la Licorne, a masterwork of the Middle Ages, and a priority on the Paris musuem circuit. Musée de Cluny.
©2012 P.B. Lecron