The symptoms of a sādhu are that he is tolerant, merciful and friendly to all living entities. He has no enemies, he is peaceful, he abides by the scriptures, and all his characteristics are sublime.
A sādhu, as described above, is a devotee of the Lord. His concern, therefore, is to enlighten people in devotional service; that is his mercy. He knows that without devotional service, human life is spoiled. A devotee travels all over the country, from door to door, preaching, "Be Kṛṣṇa conscious. Be a devotee of Lord Kṛṣṇa. Don't spoil your life in simply fulfilling your animal propensities. Human life is meant for self-realization, or Kṛṣṇa consciousness." These are the preachings of a sādhu. He is not satisfied with his own liberation. He always thinks about others. He is the most compassionate personality toward all fallen souls. One of his qualifications, therefore, is kāruṇika, great mercy to the fallen souls. While engaged in preaching work, he has to meet with so many opposing elements, and therefore the sādhu has to be very tolerant. Someone may ill-treat him because the conditioned souls are not prepared to receive the transcendental knowledge of devotional service. They don't like it; that is their disease. The sādhu has the thankless task of impressing upon them the importance of devotional service.
Sometimes devotees are personally attacked with violence. Lord Jesus Christ was crucified, Haridāsa Ṭhākura was caned in twenty-two marketplaces, and Lord Caitanya's principal assistant, Nityānanda, was violently attacked by Jagāi and Mādhāi. But still they were tolerant because their mission was to deliver fallen souls. A sādhu is merciful because he is the well-wisher of all living entities. He is not only a well-wisher of human society, but a well-wisher of animal society as well. The word sarva-dehinām refers to all living entities who have accepted material bodies. Not only does the human being have a material body, but other living entities as well. The devotee of the Lord is merciful to everyone—cats, dogs, trees, etc. He treats all living entities in such a way that they can ultimately get salvation from this material entanglement. Śivānanda Sena, one of the disciples of Lord Caitanya, gave liberation to a dog by treating the dog transcendentally. There are many instances where a dog got salvation by association with a sādhu, because a sādhu engages in the highest philanthropic activities for the benediction of all living entities. Yet although a sādhu is not inimical toward anyone, the world is so ungrateful that even a sādhu has many enemies.
What is the difference between an enemy and a friend? It is a difference in behavior. A sādhu behaves with all conditioned souls for their ultimate relief from material entanglement. Therefore no one can be more friendly than a sādhu in relieving a conditioned soul. A sādhu is calm, and he quietly and peacefully follows the principles of scripture. A sādhu means one who follows the principles of scripture and at the same time is a devotee of the Lord. One who actually follows the principles of scripture must be a devotee of God because all the śāstras instruct us to obey the orders of the Personality of Godhead. Therefore sādhu means a follower of the scriptural injunctions and a devotee of the Lord. All these characteristics are prominent in a devotee. A devotee develops all the good qualities of the demigods, whereas a nondevotee, even though academically qualified, has no actual good qualifications or good characteristics according to the standard of transcendental realization.
(Śrīmad-Bhāgavatam 3.25.21, Translation and Purport)