As a purely theoretical matter, to ascertain the original understanding of a law, at the most abstract level, you should consider how the provision was understood at the time of its enactment. That understanding can be informed by a host of things–including, but not limited to the text of the law, the history, contemporary understandings of the text, legislative debates, statements of people in the public, etc.
Though there seems to be one important conceptual limitation. I would think that any such inquiry would have to be temporally constrained, at the latest, by the date of enactment. That is, stuff that happens after the law is enacted could not affect the original understanding of the law. Perhaps that post-enactment history can be used to understand contemporary understandings, or to make policy arguments as to how the law should be interpreted. But positively, how can things that occur after the the enactment of the law in the future inform what the law meant in the past?
In Originalism and Sex Discrimination, Steven Calabresi and Julia Rickert make an unconventional argument–that original understanding of Section One of the Fourteenth Amendment prohibits discrimination on the basis of sex. They contend that a “a legal text can do more than its drafters imagined and that its scope can be affected by subsequent legislation.” Specifically, the Nineteenth Amendment (enacted in 1920) affects the original understanding of the Fourteenth Amendment (ratified in 1868).
I do not aim to take exception with the authors’ historical research. Rather, I focus on their proposed method of originalism, how it comports with the numerous strands of originalism in the academy, and how it sounds more strongly in the Supreme Court’s faint-hearted originalism jurisprudence.
In some work in the near-future, tentatively titled “Originalism at the Right Time?” I will compare and contrast the methodologies Calabresi & Rickert rely on.