Their pronunciations are so similar that many people pronounce them identically. However, "flour" has only one syllable and "flower" has two. I know some people who pronounce "flour" very carefully to ensure that it is one single flowing syllable (in fact, one of them says it so that it sounds a little like "flah"!). There is a syllable dictionary at www.howmanysyllables.com which you can use to check out the answers to such queries too (note that I only found that site today. The few words I checked on it were fine but I can't speak to its accuracy in all areas).