If they used the same setup as the LendingClub survey mentioned in the article, it was self-reported. The other notable thing I saw with the LendingClub survey (other than it being sponsored by a business that probably benefits from people living paycheck to paycheck, or the perception that lots of people live that way) was that the majority of the people who self-reported as living paycheck to paycheck also said that they had enough money on hand to cover a $1,000 emergency without having to borrow money or carry a balance on their credit card... So I'm kinda skeptical of the results.

Edit: it looks like the full Willis Towers Watson report defines paycheck to paycheck as "using most or all of their monthly income for expenses".