In 5E you can't cast any spell as ritual. Only the ones tagged with "ritual" and "Shield of Faith" isn't one of them smile
Yeah, I know - but the point your cleric probably has more spell slots available because they have not been casting Shield of Faith or Protection from Evil and Good as often as they would in 5e. So they have more spell slots to spare to cast Detect Magic with a spell slot and not as a ritual.
