First of all, a 'Guru' must have credibility. A couple of examples of gurus or self-proclaimed gurus who lost all credibility: Osho and Kryiananda. Credibility is defined by a set of conditions which must be satisfied
Some might say that credibility may be acceptable to some that are satisfied "good enough".
For example, a single perceptible negative statement made by someone about a guru or spiritual person, might be enough to hurt credibility. But some may find a statement like that does not affect their own overall satisfaction. Maybe due to the many positive things they have experienced, a negative here or there is OK.
But back to "How do we know if any guru or any scripture is truth?"
Some guru's may have had a few mishaps, but does that make what they say is truth, an untruth? I know I hear things and yes it does hurt credibility.