Or you notice that your bridge/euchre/canasta game is extremely white (and in some places, also somewhat asian). You might ask the one black pair you see where else they play cards, get an invite to a spades tournament, where you look around and are the only non black person in the room.

If you do this a bunch of times, you'll notice large non-racial demographic differences between each pairwise-related groups and you can pick through the differences to try to figure out which ones are causal.