There is only one proof of ability — action.

[Für das Können gibt es nur einen Beweis: das Tun.]

Marie von Ebner-Eschenbach (1830-1916) Austrian writer

Aphorisms [Aphorismen], No. 91 (1880) [tr. Wister (1883)]
    (Source)

(Source (German)). Alternate translation:

There is only one proof of ability: doing it.
[tr. Scrase/Mieder (1994)]