Some of the earlier examples of Flow Blue do not have a backstamp. Also, many of the early designs featured an Asian motif as Chinese porcelain was very popular and expensive. The producers sought to copy Chinese designs. That being said, some reproductions do not show a maker's mark.
Look for the smudged edges of the design to see if it's Flow Blue. One reproduced series has no bakcstamp, and the background white has a greenish cast.
Your best bet is to consult the books suggested. When searching in a book or online, make sure you take a close look at your piece. Are there flowers, vines, leaves, landscapes, birds, bridges, or other images? Measure the size of your piece as well. Closely compare familiar images. Sometimes an extra (or one less) image in the design shown in a book may indicate that you have a reproduction.