What are the original qualities of the sadhu? One is single-pointed devotion to the Supreme Lord and another is giving up varnasrama-dharma and bodily relations for the service of Sri Krishna. If you do this, then you are doing good to all. That is a sadhu. Another of the original qualities of the sadhu is that the sadhu likes to hear and speak only about Krishna:

mad-asrayah katha mrstah
srnvanti kathayanti ca
tapanti vividhas tapa
naitan mad-gata-cetasah
(Srimad Bhagavatam, 3.25.23)

Why does the sadhu wish to hear and speak only about Krishna? When the sadhu does not have the company of his beloved Supreme Lord, how can he live? How can he survive? He survives by singing the glories of the Supreme Lord, always hearing about and remembering Him. That is the life and soul of the sadhu. No sadhu can survive without hearing and speaking about Krishna. It is like water to a fish. A fish cannot remain without water. Similarly, without hearing about Krishna, without speaking about Krishna, without remembering Krishna, a sadhu cannot survive. That is his very life. That quality must be present in him. Having separation grief, he cannot remain without speaking about Krishna. That quality you must clearly be able to see: is whether or not he has the taste to hear and speak about Krishna's Name, Form, Attributes, etc.

The sadhu cannot speak for money or some other ulterior motive. Then it is not hari-katha, as it is not for Krishna:

"You have given up everything. You have taken shelter of the Supreme Lord, but you have no belief! The Supreme Lord is maintaining and sustaining the whole universe,even all the beings who are averse to Him. If anybody takes shelter of Him, will he not be protected? Do you not have such belief? You have to take money for giving Him service? Hearing is devotional service. Speaking for the satisfaction of Sri Krishna is also devotion."

"But, if we take no money, then how shall we live?"

"Then let us die! If noone gives us food for our service and we die, then let us die! What is the use of having this sort of belief? God is maintaining the whole universe. He will not maintain me? Why should I ask for this?"

If a person goes to another's house and starts working for him, without demanding anything, how long will the master of the house remain silent? Two or three days go by without feeding him. The master thinks, "He is working for me. How can I eat without giving him something?" That person might be a very bad person, but the master does not consider this. If anybody acts on behalf of the Supreme Lord, will He not protect him?

The Supreme Lord is All-Powerful. He maintains infinite brahmandas but does not give these worldly things to the devotees? So why should you accept money for speaking about the Supreme Lord. If you hear that sort of speech, then that person is speaking about the end, and that end is money.