Christians, if no one is good but god...?

how do you differentiate between who you trust and who you don't? like, let's say you're hiring a babysitter to watch over your kids. is your choice to hire one person over another purely random? or do you look for some good qualities to hire a person? would a convicted child molestor be just as appropriate to hire as let's say, good ol' aunt edna? i understand saying nobody is perfect, but to say no one is good is just stupid.