Oh, come on, surely you know the answers to these. Nonetheless, I'll humor you:
How does one know if another is mature enough to give truthful and honest feedback?
See how they provide feedback to/about other people. How accurate are they? Have they been proven wrong? What is their track record? If it's consistently good, then that's kind of a really strong voucher, no?
Furthermore, how does one discern if said feedback is actually truthful and honest?
Same as above; did you agree with their assessments of others based on the info you yourself have of those subjects? Can you verify that everything, or at least most of what, they said is correct? If they're consistently exhibiting honesty, then why is there any reason to doubt it with you?
You neutrally build a case proving or disproving their maturity and trustworthiness. It's almost scientific!