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.

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.

It was not exactly the career Mr. Rosenthal envisioned when he was growing up in New Jersey. It is one, he says, that offers a unique vantage point from which to look into questions of identity and ethnicity that go to the heart of who we are as a city, and as people.

It turns out the man behind the "Who's your daddy?" truck is more Oprah than Maury.

Mr. Rosenthal said he had worked in health care marketing for years before having what he described as a big blowup with an employer in 2010. So he struck out on his own.

He bought an RV and refocused his business on drug testing for employers.

"I put this big cup of pee on the side of the truck," he said.

It did not have the desired effect. The truck's arrival often signaled to employees that a random drug test was imminent.

"People would see the pee truck coming and they were out of there," he said.

As DNA testing became more widely available, he shifted his marketing. He had five employees at the time; they voted on the best slogan. He was the only one who favored "Who's your daddy?"

But it was his truck.