Many good newer sources will summarize the historiography of the topic in the introduction. This will give you a sense of which major older sources they're in dialogue with. Reading older sources in tandem with newer ones is generally helpful for understanding how the field got to where it is today, and for understanding which arguments and perspectives newer sources may be building upon, refuting, or revising. Sometimes reading the older sources is helpful for understanding why a newer source puts particular emphasis on certain topics or arguments, as well. I would say that it's best to start with modern, highly-reviewed overviews of the topic, and use their introductions, acknowledgements, and especially their endnotes/footnotes to fill out your understanding of what older texts they're drawing on, as well as what the more recent specialist literature is like.