Guru is not a physical form. Guru is energy, the medium through which gyan flows to the shishya or disciple. If you take the shishya to be an ice-cold stone and gyan to be boiling hot water, then the guru pours this hot water onto the cold stone gradually, drop-by-drop, lest the stone cracks with the heat. Guru takes full responsibility of the shishya, monitors him every second. Guru literally carries the shishya on the journey of evolution. He is not someone who does sweet talk or makes arbitrary promises to gain your support… A guru needs nothing from you; it is you who needs him. He is a mirror that will show you your true self and helps you elevate yourself, provided you want to rise.
The first sign of a guru is vairagya or detachment. As soon as you come in the proximity of your guru, you experience the same vairagya within -- an indication that you are in the right place. Rudryamala, describing the qualities of a guru, mentions that he has no drawbacks, is free of vices, with impeccable character, following dharma and spiritual practices and devoted to his guru.
A guru is still in the yamas and niyamas of Sage Patanjali. Anyone who follows the yamas of satya, asteya and aparigraha will never charge fees to teach yoga. Anyone who follows the yama of brahmacharya would exude the glow that comes with the vrat. A guru exudes what he talks. He possesses phenomenal energy, radiance and attraction associated with yoga. A shishya experiences this glow and attraction in the guru’s company and feels good. Just like a surya sadhak exudes the brilliance of sun, similarly a shishya radiates the glow and attraction of his guru. As you focus on the guru, gradually you start acquiring his qualities and after some time your form starts resembling that of your guru. So you need to be sure of what you follow, because you eventually become what you follow.
Rudryamala says, for anyone to be a guru, he must be of noble character, preferably born in a respectable family, of auspicious appearance, endowed with intellect, calmness of mind, and all good qualities, have a good and moral conduct, be pious and chaste and have control over the senses, be proficient in mantras and ever-engaged in japa and devoted to the worship of ishta devta. A mother is considered as the best guru for a son or daughter since there exists a pre-established connection between the two. Initiation by mother of a child is said to give eightfold results than what is ordinary.
The topic of Sanatan Kriya deals with the concept of guru and maintains that when a being finds a guru, there are certain indications for it. The first is that there occur certain changes in your body; it starts looking the way you always desired. Secondly, any imbalance or disease in the body disappears. You cease to fall ill. The third indicator is that you start experiencing energies that run Creation.
One must not rush into making a guru; you have to be absolutely sure. Because once you call somebody your guru, you must walk the path shown by him with full faith and devotion. Anything less, any doubt or change of path thereafter amounts to disrespect of the energy of guru. The journey, which begins when you meet your guru, requires continuous and dedicated practice which when followed as niyama culminates into yoga.