Unfortunately the book doesn't elaborate on how this was done. My Google-fu has only been able to refine the date to 1525 via a Google Books scan of Religion and the Decline of Magic: Studies in Popular Beliefs in Sixteenth and Seventeenth-Century England by Keith Thomas.
So I Googled "divination by frog" instead, and came up with Occultopedia.com, which reveals that the proper name for frog divination is batrachomancy, which also covers toads and newts. Frogs croaking more than usual is a sign of bad weather to come, and to meet a frog means you'll come into money.