How you know what your Guru preaches you is the ultimate truth.+

How you know what your Guru preaches is the ultimate truth. Simply because others are following you also accepted whatever he preaches is the ultimate truth. 

If you are searching for the truth then there is no need for Guru. You need a Guru if you are searching for something else other than the truth of your existence. If you are searching for truth then you do not get entangled with Gurus dead or living because it will block you from self-realization.

That is why  Bhagavan Buddha said: ~ “Believe nothing because a wise man said it, Believe nothing because it is generally held. Believe nothing because it is written. Believe nothing because it is said to be divine. Believe nothing because someone else said it. But believe only what you yourself judge to be true

The Guru is useless so long as the ultimate truth is unknown, and Guru is equally useless when the ultimate truth or Brahman has already been known.

A Guru is needed in the religious and the yogic path.  There is no need for a Guru to acquire Self-knowledge or Brahma Gnana or Atma Gnana.

Bhagavan Buddha: - There are only two mistakes one can make along the road to truth: not going all the way... and not starting.

Yoga Vasistha says: ~ “Self-knowledge or knowledge of truth is not had by resorting to a Guru (preceptor) nor by the study of scripture, nor by good works: it is attained only by means of inquiry inspired by the company of wise (Gnani). One’s inner light alone is the means, naught else. When this inner light is kept alive, it is not affected by the darkness of inertia. 

Swami Vivekananda said:~   “You have to grow from the inside out. None can teach you, none can make you spiritual. There is no other teacher, but your own Soul.”

Self–knowledge or Brahma Gnana or Atma Gann is the knowledge of the Supreme Spirit or God while religious Knowledge is lower knowledge meant for the ignorant populace.

Guru is needed to guide the ignorant populace who are sticking to the religious and yogic path.

Modern Guru and yogis propagate love, bliss, Samadhi, and grace; all these belong to duality limited to form, time and space. Thus, whoever is intoxicated to dualistic knowledge is unfit to acquire ‘Self’-knowledge or Brahma Gnana or Atma Gnana because Self’-knowledge or Brahma Gnana or Atma Gnana is based on the Nondualistic base. Thus all dualistic knowledge Instead of helping, they obstruct.

It is necessary to realize that religion, yoga, and the other paths and practices based on the waking entity (ego), which is the false ‘Self’ within the false experience, is false knowledge. The true path is the path of wisdom, which is based on the formless Soul, the real ‘Self’.

Remember:~

One has to test the truth in this world, not in the next world. There is no proof other than blind belief based on the scriptures, belief is not the truth. So, doctrines are not the means to realize the ultimate truth. There is no need to follow them.

Sage Goudpada says ~  "The merciful Veda teaches Karma and Upasana to people of lower and the middling intellect while jnana is taught to those of higher intellect.

So, they clearly indicate rituals and theories are not meant for those who are searching for higher knowledge or wisdom. The path of wisdom is the only means.

All the orthodox Advaitins indulge and immerse themselves in a ritualistic oriented lifestyle and follow the path of karma and Upasana which is meant for lower and middling intellect and not for realizing the Advaitic truth.

Mundaka Upanishad:~ The rituals and the sacrifices described in the Vedas deal with lower knowledge. The sages ignored these rituals and went in search of higher knowledge. ... Such rituals are unsafe rafts for crossing the sea of samsara, of birth and death. Doomed to shipwreck are those who try to cross the sea of samsara on these poor rafts. Ignorant of their own ignorance, yet wise In their own esteem, these deluded men Proud of their vain learning go round and round Like the blind led by the blind.

Many chose these orthodox scholars as their Gurus. But these Gurus are good to learn the conceptual Advaita meant for those Orthodox who believe their conduct-oriented lifestyle leads to Moksha [liberation]. But the theistic Advaita is not the means to acquire Self-knowledge or non-dualistic or Advaitic wisdom. 

Those who are seeking the truth have to do their own homework in order to acquire Self-knowledge or Brahma Gnana or Atma Gnana. :~ Santthosh Kumaar