That is a case of applying common sense, if you are flying in an area where display practice is NOTAM'd then talk to them, even if you are talking to people it does not mean you relax your lookout though - in fact often these will be areas where traffic is busier so lookout needs to be, if anything, a higher standard than in open areas.
Common sense is not commonly the same for everybody, if even existent.
How do you find out how? Quite rarely they publish their COM frequency.
Always listen, ok, but mandatory increase of gaggle in the air, no.