Is there a frequency comparator device?
A retriggerable one-shot set to the pulse width of threshold frequency period will stay active ON. Then any interruption greater than this period will time out.  If you want a smoother operation, a LPF on this output will delay f>set f and filter out a 1 or more cycle dropout.

This can be achieved with a 1/2 dual ‘HC123 IC very easily. a Pot and series R with cap can tune to &lt;1% if large tolerance parts are used and is very stable with Vdd and temp.  

Using two of them you could choose 2 frequencies like +/- 10% if that was useful.