Best bet is to check the acknowledgments page. Not every author thanks their agent, but some will. Actually more common, IME, in NF because you typically expect a longer acknowledgments page where the author will thank librarians and others who helped with the research. Most will mention their agent, too.
If that fails, try checking an author's website, especially the contact or events pages. A lot of event booking for authors go through agents, so you can sometimes find out that way.
Twitter bios, too, though I find that tends to be newer authors.
If all else fails, you can just Google and see what comes up. The phrase "[TITLE] sold by at" might net you some results from Publisher's Lunch and such that will mention the agent involved.