I prostrate respectfully to the spiritual teachers,
Eyes that see all the infinite scriptures,
Who with skillful means moved by compassion
Clarify the supreme entryway for the fortunate traveling to liberation.
A valid Teacher must have five qualities:
(1) Unmistaken knowledge of all objects to be abandoned
(2) Complete knowledge of the methods for abandoning them
(3) Unmistaken knowledge of all objects to be practised
(4) Complete knowledge of the methods for practising them
(5) Revealing all this to others with the motivation of compassion
If we find such a teacher we can place our unreserved trust in him or her. I guess the problem is knowing whether a Teacher has these qualifications or not and that depends upon our own investigation.
A fully qualified Mahayana Spiritual Guide is someone who possesses ten special qualities. According to Ornament for Mahayana Sutras
(1) A mind that is controlled by the practice of moral discipline.
(2) A mind that has become peaceful and undistracted through the practice of concentration.
(3) Reduced self-grasping through the practice of wisdom.
(4) Greater knowledge than the disciple.
(5) Delight in teaching Dharma.
(6) A wealth of scriptural knowledge.
(7) A deep and stable realization of emptiness.
(8) Great skill in explaining Dharma.
(9) Compassion and love for his disciples.
(10) Enthusiasm for teaching Dharma, being free from discouragement or laziness.
It's advisable to take time to check out any potential Spiritual Guide against this list of qualities over time.