From the Bhagavad Gita’s descriptions of a man of perfection (Chapter II), we know that a man firmly established in wisdom is tranquil, and his equipoise is never disturbed even when he invests his entire energy in the service of mankind.
The Upanishads summarize the qualification of a teacher in two terms: Shrotriya (one who is a master of scriptures) and Brahmanishtha (one who well established in the experience of Truth).
Without knowledge of the scriptures, the teachers will not be able to convey his wisdom to the disciples. But a mere bookish knowledge is not sufficient. The words coming from an individual can gather wings only when they spring from a heart soaked with sincere subjective experience.
However, to be a Guru or teacher, he must have two more qualifications. His behavior in the world must be perfect, since we as students when admiring the teacher will be tempted to imitate him in all his external habits. If his behavior is not perfect, it is possible that we will copy his bad habits and thus ruin our chances.
Secondly, a Guru or teacher must have large-heartedness and be flowing with kindness and patience. This is necessary since in the early stages the students will revolt against new concepts that conflict with their present understanding. To weed out the mind to replant new ideas is a very painful operation, this can be achieved only when the teacher has infinite patience, endless love, and supreme affection.
When these qualifications are not there, the Guru is not a true one. A teacher is known by his own disciples, just as a good musician is known only by true students of music.
Students who have a spiritual urge and have practiced a little will instinctively recognize a teacher.
Our Guru is he who inspires us to live nobler life, and in whose presence we feel elevated. When we compare our life with his we feel ashamed of our own weaknesses; at times of burning passion, by remembering him, we feel cooled down.
In fact, the true Guru to all of us is the Lord, and the Lord of our heart talks to us very often through His chosen deputy among us, and we revere and worship Him as manifesting through the individual. No individual mortal is ever a Guru. The Lord alone is the Teacher, everywhere and at all times.
(Source : Swami Chinmayananda Reader - The Penguin – Page 159-160)