This is by no means an easy thing to do and the methods for achieving an accurate idea of how gain is being reduced are very dependant on the gain element as well as the detection system.
It could be FET, VCA, optical, chopper......
Generally you have to find the feed to the gain element and see if you can take a sniff off it to give you something meaningful to send to a display. This isn't always possible. I've seen some circuits (Joe Meek comp for example) where they've used a comparator to compare the original signal to the limited one to show the difference on a meter
I've no idea what the Ross uses in their circuit.
I have used a simple schmitt trigger in the past to give a single LED indication of when gain is being reduced. This was for an optical compressor circuit from a Lexicon - something I've been thinking of doing here as a project. But you have to do a fair bit of work to determine the values - it's not just a matter of finding a test point and sending it to a meter.
Good luck !