If any piece of Flow Blue is in excellent condition it will be worth something. Although prices are not as high now as they were in the past, when everyone was collecting everything like crazy, there will still be some value.
Now that older people are downsizing and younger people prefer mid century modern, demand for Flow Blue China has decreased.
Value can change quickly and even fairly recent price guides can be off. The is a difference in regional markets as well. Expect a higher value for unusual items or items with handles, lids, and spouts that can easily break.
More plates, soup bowls, and dessert plates were produced so they will be cheaper than things like tea pots, lidded soup tureens, or things that came one to a set.
You can research your pieces in a book. Older books are good for identification but do not reflect current value. For information, try
"Gaston's Flow Blue China : A Comprehensive Guide Identification and Values" by Mary Frank Gaston, 2005.
The online site and collectors association, the Flow Blue International Collectors Club is a font of information.