спокойно на душе оттого, что кто-л. знает(Many families will now have the peace of mind of knowing that their kids are learning in a new facility that also has child care spaces next door. (Premier David Eby)ART Vancouver)
понимать толк в чём-л.(The expression "know one's onions" means to be very knowledgeable or experienced about something, to know a great deal about a particular subject. (Collins Dictionary) The origin is Cockney rhyming slang: onions = onions rings = things.: He knows his onions. -- Он знает / понимает толк в этих делах.ART Vancouver)
быть в курсе последних событий(scoop is information about something that is currently important or happening or that is interesting to many people: She always knows the scoop on fashions.Yerkwantai)