Chemistry: The Molecular Science (5th Edition)

A water soluble substance will break into cations and anions when placed in water. Thus, in order to make a guess whether or not it is soluble, see if you can find an anion and a cation in this substance. At this point in the course, if you can find a cation and an anion, then it will most likely be soluble. We see that there is $Na^+$ and $F^-$, so it is soluble.