What Is A True Guru versus a spiritual teacher?
An ordinary spiritual teacher simply gives you information that he/she may have never tried. For example, a spiritual teacher may provide steps to any number of spiritual practices for which he/she may not have an extended period of experience to support. Spiritual teachers often share approaches that they themselves have not mastered over an extensive period of time. They may also provide intellectual understandings of approaches, philosophy, etc. that they have no direct experience of and have never really tested in the laboratory of their own existence. Essentially, spiritual teachers are information providers of intellectual knowledge that is, usually, already available and popular.
A true Guru, on the other hand, is one who has completely mastered what he/she teaches, over an extended period of time, by way of direct, ongoing experience of what is being taught. Such Gurus do not simply handout intellectual knowledge. They both teach and give the direct experience of what is being taught, by way of the ability and authority to grant Grace. So, a true Guru has exceptional spiritual power with which to lead you to the experience and unfolding application of a true spiritual path. As such, a true Guru leads by example and is a perfect and obvious example of what he/she instructs you in.
The purpose of taking such a spiritual master is to be led in the spiritual practices, methods and instruction for realizing the goal of the spiritual path that the Guru instructs you in. Therefore, an authentic Guru will always be able to clearly elucidate to you the goal of the spiritual path, along with the proved means for attaining that goal.