Our connection with an infallible lama indicates great merit and strong prayers of aspiration in past lives.
However, a terrible spiritual tragedy results if we meet and follow a false teacher who distorts the teachings and does not base spiritual instruction on pure lineage transmission, who merely pretends to have realization, whose concern for followers is motivated by self-interest. Such teachers waste their followers’ opportunity for spiritual development in this life, betray them when they die, and undermine the merit they need to find a spiritual path in the future. If treated by an incompetent, fraudulent physician, a patient can expect declining health or a loss of life. If guided by a false teacher, a student can expect to lose spiritual well-being for this and many lifetimes to come.
Some enlightened lamas manifest wisdom in unconventional ways; some charlatan lamas seem serene and wise and have fine reputations. How can we know the difference?
We check their lineage—true lamas revere their lineage and have served their own teachers with exemplary devotion. We check their motivation, their good heart. Is their intention really to benefit sentient beings? Can we feel compassion underlying their actions? Again, wrathful activity can be carried out with unconditional love and compassion; peacefulness can be hypocritical. We check back. Do we experience a greater clarity as a result of the lama’s words and actions? Are our mind’s workings illuminated? Do we gather impetus to practice and correct our conduct? A true lama can bring about moments of positive transformation through the skillful means of dharma.
A false lama merely manipulates our spirituality and reinforces poisonous emotions and deluded tendencies. Someone posing as a lama, but devoid of pure lineage and pure motivation, devoid of the qualities that arise from authentic meditation, resembles something foul wrapped in brocade. The eye may be deceived, but the nose can smell it.
Once we have found an infallible lama, we should hold him or her as dear as our own breath. Now we have access to a treasury of spiritual attainment. In our interactions, we try to see the lama through less ordinary eyes, cultivating the pure view that the lama’s activities of body, speech, and mind remain inseparable from Guru Rinpoche.
Although in a relative way we might find fault with the lama’s human foibles, this tendency to belittle and criticize undermines our own spiritual development. The lama has intentionally accepted the limitations and suffering of human rebirth, yet abides in the recognition of buddha nature. Outwardly, to guide us, the lama may act like one of us; actually, he or she is completely different. If our obscurations make us too nearsighted to perceive the buddha manifestation in the lama’s outer display, at least we must refrain from any immature, arrogant judgments. Otherwise we may block our avenue of liberation.
Source: Tromge, Jane. Ngondro Commentary: Instructions for the Concise Preliminary Practices of the New Treasure of Dudjom. Compiled from the Teachings of His Eminence Chagdud Tulku. Junction City, CA: Padma Publishing, 1995.

We check their lineage—true lamas revere their lineage and have served their own teachers with exemplary devotion. We check their motivation, their good heart. Is their intention really to benefit sentient beings? Can we feel compassion underlying their actions? Again, wrathful activity can be carried out with unconditional love and compassion; peacefulness can be hypocritical. We check back. Do we experience a greater clarity as a result of the lama’s words and actions? Are our mind’s workings illuminated? Do we gather impetus to practice and correct our conduct? A true lama can bring about moments of positive transformation through the skillful means of dharma.
(Jane Tromge. Ngondro Commentary.)