There are so many conceptions of sadhu. But sadhu is described in the Bhagavad-gita by the Lord Himself, who is sadhu. He said api cet su-duracaro bhajate mam ananya-bhak, sadhur eva sa mantavyah [Bg. 9.30]. Sadhu is one who is unflinchingly engaged in loving service of the Lord. He is a sadhu. That is the test, who is a sadhu.
Lecture on Bhagavad-gita 10.8, New York, January 7, 1967