system
A chat on 4chan
human
I accepted my guru through a number of considerations. 
First, what do they say? I read their works, their letters, saw how they matched to those I already accept as pure in my tradition. I listened to their lectures to get an idea of their mentality, their relationship to scripture, to Krishna, and to their own guru. It gave me an idea of how they might teach, scold, instruct, and otherwise communicate spiritual wisdom.
Second, what are they like? I asked people who had interacted, I heard past stories and current considerations of personality. I asked community leaders and those personally close. I was fortunate to meet them and observe their activity when not 'on stage'. When I observed this person, was I convinced that they acted in service to Krishna - or at least sincerely trying and mostly succeeding.
Third, what are the results of their instruction? I met with the disciples of my guru. I was amazed by their humble intensity, how lovingly strict and high-class their learning, their sadhana, their attitudes were. How does the temple Krishna gave them to care for run?
gpt
And last, of course, is that inner feeling Paramatma gives you where you KNOW this person is your eternal connection to Krishna and the discipline line. That maybe others are more knowing, more prominent, perhaps even more regulated in sadhana, but they are not your guru. Guru is like marriage, bit even more so. There is something to it that is more than just these identifiable factors.