That's kind of the thing. You don't. Arguably ones that have been running for a long time could be trusted, but really even long time running mints could eventually rug due to technical problems or whatever.

Cashu is interesting, but one of my biggest beefs with it is the whole custodial trust aspect. Imo The whole point of Bitcoin and even Lightning is to be self-sovereign with your transactions and not trust a custodian. And cashu, while it is really cool, adds back that custodial trust. (And I guess to be fair, most of people using lightning are using custodians so /shrug)