The same question was asked to Lahiri Mahasaya by one of his devotees:
How to identify true saint?
He replied:
Those whose can place the tongue has reachedabove the soft palate and into the nasal cavity are real saints.
The above mudra is known as Khechari Mudra which Paramahansa Yogananda described as:
Through the performance of Kechari Mudra, touching the tip of the tongue to the uvula, or "little tongue," (or placing it in the nasal cavity behind the uvula), that divine life-current draws the prana from the senses into the spine and draws it up through the chakras to Vaishvanara (Universal Spirit), uniting the consciousness with spirit.
Purana Purusha, Page No. 132
