There's a question about whether one would be the third wheel or marginalized, and near-term you wouldn't be able to answer that with 100% confidence, but you might get sufficient confidence.

Early good signs include (if pre-seed) all three of you getting the same equity deal.

Then I'd consider things like how candid, honest, and thoughtful they are.

But if the startup's premise or initial execution looks ethically questionable, run away.

(For example, I wouldn't trust a situation where the other two might as well say "We're exploiting users or gig workers to their disadvantage, ignoring regulations, self-dealing, doing pretty much anything in cryptocurrency/blockchain, and/or building an investment scam house of cards that we'll cash out of before anyone realizes... but the two of us totally wouldn't collude to screw you, even though we'd have similar incentive to also do that.")