you cant tell if a car is running rich by listening unless is flooding and revving slow low down but lears up after rpm climbs.
you can however smell if you are running rich.
in the morning after starting you car ,stand behind the exhaust, there should be a slight hint of gas (smell) it would be really bad if you are running rich and when the car warms up during the day (if you stand behind the exhaust and it still smells the same way (slight hint of gas) then you are running a little rich.
extreme rich is when the exhaust looks slightly or dark black, car pops and backfires alot, black residue on the inside of exhaust, thick and wet(not necessarily dry and powdery). super rich when cars revs at high rpm plugs are fouled( black) exhaust is black, and falls emmediatly to ground in little balls after exiting muffler.
just a guide as to some of the signs of running rich.