You can count the incoming tuned carrier, if the counter is sensitive enough, disagreeing with what mblack said.
One of the very best frequency counters to add to a radio, as an "add-on" was made by CCI. It came as a kit, see this thread:
https://antiqueradios.com/forums/viewto ... 8&t=394544There is enough data there to build one.
You can configure it to measure frequency directly, or measure the L/O in a superhet radio with an offset equal to the IF frequency.
It was very sensitive at its input, but I also added an input pre-amp to make it work on a sniff of signal of a small fraction of a millivolt with very low coupling, so it wouldn't affect the circuit being monitored.
There are now lots of frequency counter modules on ebay with uP's that look like good value, but none are as good as this old CCI counter, and most require a substantial input voltage > than a few hundred millivolts pp to count reliably.
In between these two extremes of a modern firmware based counter with a uP, and a counter based on plain digital logic IC's (like CCI's counter) is a single chip counter, dedicated for the task. Basically these are digital logic all in one package with the LED display drivers in there.
Intersil specialized in these counter IC's, a typical example being the ICM7208, which greatly simplified making a counter, these where used in Philips RF generators that went to 125MHz, for example the model PM5326:
https://www.semiee.com/file/EOL2/Intersil-ICM7208.pdfThese are dead easy to use (but for your application would require a pre-amp), I collected many of these IC's years ago as I could see the beauty of them, and they are mostly superior to more modern firmware uP based counters without any disrespect to the programmers of more modern ones.