Guru and Disciples
"Srila Rupa Gosvami therefore recommends, sri-guru-padasrayah: one must seek shelter at the lotus feet of a pure devotee who can be one's guru. Caitanya Mahaprabhu advises that a guru is one who strictly follows the instructions of Bhagavad-gita... A juggler, a magician or one who speaks nonsense as an academic career is not a guru. Rather, a guru is one who presents Bhagavad-gita, Krsna's instructions, as it is. Sravana is very important; one must hear from the Vaisnava sadhu, guru and sastra."
"We must take instruction from the spiritual master. Adau gurvasrayam, sad-dharma-prcchat. Who requires a spiritual master? One who is inquisitive of sad-dharma, not asad-dharma. ...A man requires a spiritual master when he's inquisitive to know about the transcendental subject matter. ...To accept a spiritual master is not a fashion. Just like we keep a dog, pet, similarly, if we keep a spiritual master, pet spiritual master, to get sanction of all my sinful activities, that is not accepting spiritual master. Spiritual master means tad viddhi pranipatena pariprasnena sevaya. You should accept a spiritual master where you think that you can surrender yourself fully, and offer ...your service. That is spiritual master."
Srila Prabhupada Lecture, 11-13-72, Vrindaban
A guru is a bona fide spiritual master who follows sadhu and sastra. Guru means abiding by the injunction of the sastra. Guru is in the disciplic succession, one who is strictly following the footsteps of the sad-gosvamis and is speaking strictly on the basis of sastra.
So many unauthorized persons are presenting themselves as guru, but one must be very careful. Guru does not manufacture any rules and regulation. He refers to the sastra. In order to know who is a qualified spiritual master, corraborate with sadhu and sastra. Sadhu confirms the scriptures and guru accepts the scripture; they are never in disagreement.
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.