My guess is a lot of it is "Credit Score By Proxy": you could pay one of the US' three Credit Score Bureaus directly, all of whom charge for the service about as much as they think they can get away with when you ask for a contract and all of whom have reasons not to trust direct relationships with them. Or you could check that the person has a credit card with any limit above $0 implying some bank is already subscribed to the credit reports on you and still chooses to be in a relationship with you.