OK, learn something new every day.
Here is something I found on the net
https://k3jls.net/sniffer.jpg, called the Sniffer, not exactly what you are looking for, but, if you used an FM radio (for the band of interest) and the input stage in the diagram connected to a frequency counter, it might work.
Full description here
https://k3jls.net/auto_ctcss.html