At the beginning, this is a random process. But it is quicker and easier than you might think because at this stage you want to avoid finding out what the site authors believe.
Instead, you want to find out what sources they used. If they don't name their sources—ditch them immediately.
So, you'll start out searching for something like The Hanging Gardens of Babylon and every possible variation, abbreviation, misspelling, and individual word. And then work through the search results and bookmark every site you use.
Your bookmark file is a useful catch-all, but as time goes by, you'll want more than that. I suggest you build an Organized Sources Page as you go.
This lets you drop new materials into the appropriate places, and add notes as you discover useful information (dates, places, problems, common mistakes you've found, etc).