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.

Over the years, he said, he has brought long-lost siblings together and told others that they were not, in fact, related. He has told men that the children they raised were not biologically their own. He has told others they were the fathers of children they never knew they had.