GURU WHO

Any thought coming from anywhere

if capable of provoking you or disturbing you or making you unsettled mentally

and

then in turn forces you to think/ponder over the existential questions like

who you are, where you are,why you are, etc. as is that has become a question of life and death for you,

then

understand that either the source (may be a person) is your Guru

or

the thought itself is your Guru (if you are sufficiently intelligent).