Almost everyone Jared Rosenthal does business with has a secret or a suspicion, something they hold close and keep confidential and do not share even with those nearest to them.

But they tell Mr. Rosenthal.

He is not a priest or a psychologist. He is not a doctor or a lawyer.

He is the owner of two trucks, each emblazoned with a slogan as blunt as it is effective, posing a simple question: “Who’s your daddy?”

The trucks — recreational vehicles that have been converted into rolling laboratories offering on-the-spot DNA testing — invariably attract stares and questions when they appear around New York City, and might seem like unlikely confessionals.

But Mr. Rosenthal, 46, said the trucks have been extraordinarily effective marketing tools, and that with DNA testing, intimate stories of intrigue and revelation are never far.