The Guru & the Disciple

The Guru is the gatekeeper who leads us directly to the lord. He is the servant of the Lord and therefore acts according to the Lord's will. Being His servant he is familiar not only with how to reach Him, but also with the various pitfalls along the way.
The Guru is the representative of the Lord and the disciple must accept him as such. Whether the Guru considers himself to be a mere instrument, a channel, a link between the Divine and the devotee is his affair. The disciple's surrender to him must be unconditional and spontaneous. He must efface himself completely, unquestioningly and gratefully.

The fundamental principle of Guruvad is the unconditional surrender to one's Satguru. Now, how will we know who is a Satguru?
Dadaji used to say if you find in a 'Sadhu' any of the following three faults, be rest assured he cannot be a Satguru. The faults are:

If he is a lustful man who has not conquered his lust or sex.
If he is a greedy man having greed for money, power, etc.,
If he tells falsehood

This is how Ma Indira Devi described Guruvad:
"In Yoga, the things that hamper us the most are the small petty movements of desire and possessiveness. Nothing is too small to prove an obstacle and nothing is too big to be overcome with sincerity, perseverence and the Guru's help, vigilance, vigilance, vigilance all the time; at every step it is essential. Whenever anything unseemingly happens we must look within. That is the difference between the worldly man and the Sadhaka. The former always looks without and finds fault with all except himself; the sadhaka looks within and tries to find the root of the trouble hidden somewhere under the subtle layers of the ego. What happens outside is just a projection of what is inside us. Nothing can affect us if we have no opening to it within ourselves. Unless there was a point of contact between the two we would not be affected."
"We often think that as soon as an aspirant enters an Ashram he becomes a saint. But that is far from the truth. The aspirants stay as human as ever; only, with Yoga, they become aware of their human failings and so have to strive to change themselves under the guidance of the Guru. But transformation of nature is a very arduous task. Constant vigilance, aspiration, prayer and ruthlessness with oneself are necessary before one can change. I am talking ofcourse of those who are sincere and want to change. The rest will always go on deceiving themselves and justifying their egoistic movements till the end of time."
"Broadly, there are four stages in the process of transformation: Vigilance, Consciousness, Aspiration and Transformation. First of all we must be vigilant about our thoughts and acts. Vigilance would make us conscious of our shortcomings. When we are conscious we aspire to change and this would lead to the transformation of our nature."