literally: 'distance tests a horse's stamina' (you can never know what someone true abilities are until after a period of time); the proof of the pudding is in the eating
Definitions (Wiktionary)
proverb
a long road tests a horse's strength; distance tests a horse's stamina; only time can reveal a person's true strength
It's in times like these that the character of a person becomes apparent.
Examples (Wiktionary)
Mandarin
游击战争没有正规战争那样迅速的成效和显赫的名声，但是“路遥知马力，事久见人心”，在长期和残酷的战争中，游击战争将表现其很大的威力，实在是非同小可的事业。
游擊戰爭沒有正規戰爭那樣迅速的成效和顯赫的名聲，但是“路遙知馬力，事久見人心”，在長期和殘酷的戰爭中，游擊戰爭將表現其很大的威力，實在是非同小可的事業。
yóu jī zhàn zhēng méi yǒu zhèng guī zhàn zhēng nà yàng xùn sù de chéng xiào hé xiǎn hè de míng shēng ， dàn shì “ lù yáo zhī mǎ lì ， shì jiǔ jiàn rén xīn ” ， zài cháng qī hé cán kù de zhàn zhēng zhōng ， yóu jī zhàn zhēng jiāng biǎo xiàn qí hěn dà de wēi lì ， shí zài shì fēi tóng xiǎo kě de shì yè ．
Guerrilla warfare does not bring as quick results or as great renown as regular warfare, but "a long road tests a horse's strength and a long task proves a person's heart", and in the course of this long and cruel war guerrilla warfare will demonstrate its immense power; it is indeed no ordinary undertaking.