I know journals give a good representative of the top historians in WWII history, ANE history and Ancient Roman. This should be the case for other fields of history. For someone to publish several works in a peer-reviewed journal, they have to be serious and very prolific in their fields. Journals are a good place to start your search; find journals that publish for the field of history you're interested in and go from there. Also, you can randomly select 10 journal articles on the subject of interest, and see who are the most cited authors across the board.

Another way to identify top scholars is to just troll the history department websites for the top universities. If you look at Harvard, Yale, Stanford, Princeton, Penn, Chicago, UW-Madison, Michigan, Berkeley and others of that caliber (don't yell at me if I left your favorite out - this is not a complete list), you'll get some good names. You'll obviously miss a lot of people, but it's a start.

As a supplement, find one historian whose work you find interesting or that others have identified as important. Go to your university or college's library catalog and find the entry for one of his or her books. There should be a "subject headings" section of this entry, hopefully with a hyperlink to click on the subjects (something like History--United States--social aspects). If you click on one, you'll get a list of other works that the library cataloguers have identified as having similar subjects (many of these subjects are standardized by the Library of Congress). Any book in this list that is published by a university press and was published in the last ten years or so is likely to be at least useful to you. Look up reviews of that book, that author's other works, and his or her career status. These will all give you clues as to how well this person's work has been received.