It is very important to check out the teacher that we are about to take refuge in or accept as our teacher, tutor and guide. The Guru guides us not only in this life but in all future lives as well which is why we have to check very carefully to see if the teacher is qualified to guide us or not. Many people these days take Guru devotion very lightly and think that they can just abort their current Guru if their Guru stops saying words that please their ears or challenges the a little for another one, since there are so many. Nobody really bothers to check anymore these days. According to the 50 verses, the criteria of a proper teacher are:
7. A disciple with sense should not accept as his Guru someone who lacks compassion or who is angersome, vicious or arrogant, possessive, undisciplined or boasts of his knowledge.
8. (A Guru should be) stable (in his actions), cultivated (in his speech), wise, patient and honest. He should neither conceal his shortcomings, nor pretend to possess qualities he lacks. He should be an expert in the meanings (of tantra) and in its ritual procedures (of medicine and turning back obstacles). Also he should have loving compassion and a complete knowledge of the scriptures.
9. He should have full experience in both ten fields, skill in the drawing of mandalas, full knowledge of how to explain the tantras, supreme faith and his senses fully under control.
you can have a very fierce teacher, a very quiet one, a famous one, an unrecognized one, but no matter what he or she needs to have the qualities listed in these 3 verses. If they have, it means they are qualified. It is not a technical thing per se, but something that we have to put our effort in to get close to the teacher and observe, or observe via his teachings and going through them and see. Another way is to observe the students.
There is nothing wrong with questioning the Guru to understand more about him but it is not right to question the Guru to challenge him or to avoid his assignments, or to discredit his Dharma, or even to hurt him. People these days are no longer aware of what is a fine line between questioning for knowledge or questioning to put down or undermine. If it is the former, the Guru will always be happy to answer but if it is the latter, one just creates the causes to never have a teacher, ever.
I have seen and met people who walk into the center, demanding to meet the Lama and the questioning the Lama all kinds of questions, and then when the answer is different than what the person expects, they will ask the question again until they get the answer they WANT to hear, not the real answer. If they dont get what they want to hear, they literally scorn the lama and put him down, either in front of the lama or behind his back. That is the kind of questioning that is "not allowed" in this context. Questioning like "Dear Lama, why do you act in such a way? I do not understand and to prevent myself from misunderstanding you, please in all kindness, explain" is obviously permissible. If the lama does not respond to that, or responds in an aggressive way, something is up with him.
This is just very, very basic logic. Not too hard to know if we are questioning to learn or to question to put down our Lama. Not rocket science, but glad that the Dalai Lama explained about it anyway.