There are six chief criteria mentioned in the authoritative Vedic scriptures that determine the authenticity of a guru.
Second, the guru must have attained the highest realization of the Absolute Truth—the personality of God—which is beyond the incomplete impersonal conception. In this way the guru will be competent to distinguish between the infinite Supreme Spirit (God) and the infinitesimal spirit souls. Fully aware of this distinction between God and the individual soul, he should engage himself as a devotee of the Supreme Lord and teach devotional service to his disciples.
The third criterion is that the guru must have received his knowledge from a bona fide spiritual master through the authorized process of disciplic succession, as explained in the Vedic literature.
The fourth is that the guru’s character must be pure: he must rigidly follow all the religious principles governing saintly behavior.
Fifth, he must engage himself and others in the glorification of the Lord as the ultimate activity for human society, and he should not divert his energies to material welfare work.
And sixth, a guru of the highest order must spread the message of God (Krishna) consciousness throughout the world.
Therefore, If a self-realised soul is able to fulfill all the above-mentioned six criteria and accordingly dispel the darkness of ignorance by following the words of authorized bonafide texts in disciplic succession, ultimately delivering conditioned living entities, that is the real meaning a Guru.