I agree. Number one would be NBI Clearance just so we can identify if the person has committed crimes before. But personally, I would prefer to get a yaya from a friend/relative's recommendation. That way, I can somehow be assured that someone knows about the yaya's identity.
NBI clearance and talking to her previous employer are musts for me. The interview is also quite crucial: https://ph.theasianparent.com/5-questions-must-ask-yaya-interview/