Detection of NaF levels in aqueous solution also containing NaCl
Depending on the equipment you have available, you could buy some $\ce{CaCl2}$ de-icer, dissolve it in water, and add it to a known volume of your solution.  Highly insoluble $\ce{CaF2}$ will precipitate out; keep adding the $\ce{CaCl2}$ solution until no more precipitation is observed.

You can then filter the precipitated fluoride from solution, rinse and dry it, and weigh it.  The original fluoride concentration can then be back-calculated based upon the mass of obtained $\ce{CaF2}$, the percent by weight of fluoride in $\ce{CaF2}$, and the original solution volume.