1. They groom themselves more often than I am comfortable with
2. I closely examine the fur of the whitest kitten and spot some suspicious specks of black dirt
3. I pluck one speck, put it on a piece of white paper, wet it with water, wait a few moments: it dissolves and becomes reddish. That's the proof! It's not dirt, but flea excrement, made from kitten blood. How strangely gratifying it is to witness the definitive confirmation of one's fears.
Now that I have a head start on identifying the problem, I should be able to control it before we get infested. I hope.