Transitive trust: If you trust Alice, and Alice trusts Bob (of Bob's Discount Bed Nets & Vaccine Research Shop), then you might trust Bob somewhat on the strength of Alice's trust.

Transitive authentication (weaker): If you believe Alice is a real person and not a spambot (because you've met her), and Alice believes Bob is a real person and not a spambot, then you might trust that Bob is not a spambot too.

Track record: If the Carter Center has been effective against guinea worm, then you have reason to believe they'll be effective against other horrible parasitic diseases in the future. (This establishes that they're competent, not that they're the optimal cause on margin.)

Reasoned community discourse: If you observe open discussion of causes, where people actively disagree with one another in good ways, change one another's mind in ways that are consistent with intelligent evaluation, including calling out mistakes or bad reasoning, you might credit the evidence presented in that discourse. (The disagreement and calling-out of mistakes are necessary to make sure you're not looking at a mutual admiration society.)

Bay Area House Party: Go hang out with a bunch of EAs and troll them into an argument over whose cause is better. (If they all agree, find a different party.)

My advice: don't over-generalize.  There's no easy solution for knowing what public sources to believe, in the face of lots of conflicting, unreliable information that's purpose-designed to get your money and attention. 

Instead, pick a few instances and dive deep - both into their referrals (people you kind-of-trust who endorse them) and into their hard-to-fake documentation (how long they've been doing their work, what are their goals and how are they measuring, what ratings or evaluation agencies say about them).  

Alternately, if these are relatively small amounts you're spreading thin (tens to hundreds a year across multiple targets), pick somewhat randomly by sector/cause, and just hope you'll mostly do some good.