Srila Bhaksiddhanta Saraswati Thakur explains which Gurus should be rejected
These are also several other important points to which we have to attend if we want to preserve the purity of communal worship. Those who occupy the pulpit must not deviate from the true principles of the religion and indulge in promulgating the concoctions of their imaginations. No one is fit to be a teacher of religion who is not really free from all taint of any ambition for popular applause or ‘self-conceit.
Thakur Bhaktivinode warns everybody to be extremely cautious in the election of the spiritual guide. The preceptor must be fit in every way to instruct one in the true principles of religion. The preceptor who initiates is to be implicitly obeyed and cannot be discarded except for two definite reasons viz. (I) if the disciple at the time of choice of preceptor “failed to choose as his guru one who is ignorant of the principles of religion and is not a Vaisnava, i e. a true devotee of the Absolute, such a guru has to be discarded for the reason that be is of no practical help ; and (2) if the preceptor although he happened to be a Vaisnava and well-versed in the principles of religion at the time when initiation as received from him, subsequently comes a professor of illusionist doctrines or a hater of Vaisnavas such a guru should also be discarded.
Thakur Bhaktivinode lays special stress on the avoidance of all association with un-godly persons. These un-godly persons fall broadly in two classes ‘viz.
(I) those who are carnally addicted to women and (II) those who are without
devotion for Krishna. There will be absolutely no spiritual progress if such aloofness is not strictly maintained despite the adoption of every other
process that is recommended by the Scriptures.
[Thakur Bhaktivinode, The Harmonist, June 1928, Vol. 26]