Are you trying to date the physical copy that was digitized, or the published book as a text?

For the former, you can look at typographical clues--typeface (font), capitalization, special characters, sizing of words on title page. You can look up the publisher to see when they existed. This will tell you whether the copy that was digitized could have been produced in the ballpark era. At that point, you have to trust that the library that loaned the book or the efile to Google--in this case, Cologne--recognized the physical paper and ink as genuine period.

As for the "text", the words and the author, you can use WorldCat to see if the book checks out to its listed publishing date. Here you go. :)

There are more specialized catalogs to use for the 15th and 16th centuries, and by country, especially for smaller pamphlet-type texts. But WorldCat is a good sweep-up that will catch quite a bit.