And there the Blessed One addressed the bhikkhus, saying: "Now, bhikkhus, I shall make known to you the four great references. Listen and pay heed to my words." And those bhikkhus answered, saying:
"So be it, Lord."
Then the Blessed One said: "In this fashion, bhikkhus, a bhikkhu might speak: 'Face to face with the Blessed One, brethren, I have heard and learned thus: This is the Dhamma and the Discipline, the Master's Dispensation'; or: 'In an abode of such and such a name lives a community with elders and a chief. Face to face with that community, I have heard and learned thus: This is the Dhamma and the Discipline, the Master's Dispensation'; or: 'In an abode of such and such a name live several bhikkhus who are elders, who are learned, who have accomplished their course, who are preservers of the Dhamma, the Discipline, and the Summaries. Face to face with those elders, I have heard and learned thus: This is the Dhamma and the Discipline, the Master's Dispensation'; or: 'In an abode of such and such a name lives a single bhikkhu who is an elder, who is learned, who has accomplished his course, who is a preserver of the Dhamma, the Discipline, and the Summaries. Face to face with that elder, I have heard and learned thus: This is the Dhamma and the Discipline, the Master's Dispensation.'
"In such a case, bhikkhus, the declaration of such a bhikkhu is neither to be received with approval nor with scorn. Without approval and without scorn, but carefully studying the sentences word by word, one should trace them in the Discourses and verify them by the Discipline. If they are neither traceable in the Discourses nor verifiable by the Discipline, one must conclude thus: 'Certainly, this is not the Blessed One's utterance; this has been misunderstood by that bhikkhu — or by that community, or by those elders, or by that elder.' In that way, bhikkhus, you should reject it. But if the sentences concerned are traceable in the Discourses and verifiable by the Discipline, then one must conclude thus: 'Certainly, this is the Blessed One's utterance; this has been well understood by that bhikkhu — or by that community, or by those elders, or by that elder.' And in that way, bhikkhus, you may accept it on the first, second, third, or fourth reference. These, bhikkhus, are the four great references for you to preserve."
Footnote: In the earlier edition of this work, mahapadesa was rendered as "great authorities." It is now known that the proper meaning of apadesa is not "authority," but "reference" or "source." Besides, from the passage it is clear that there are only two real "authorities" — the Discourses (Suttas) and the Discipline (Vinaya).
There's a canonical description of the characteristics of the Dhamma:
The Dhamma is well declared by the Bhagavā:
visible here and now, immediate,
inviting to come and see, effective,
to be individually ascertained by the wise.
I've seen some people apparently try to use that too to discriminate, using arguments like, "if it isn't sandiṭṭhika then it's not true Dhamma".