How do you find where fleas are hiding?

I had 3 cats once who had a really bad flea problem in an apartment I was living in. Before I knew for sure that there were fleas, I was suspicious because of a couple of bites and a brief glimpse of a bug. My mom told me to take white socks and run them over the cats, the carpet, etc because it would make the fleas more visible. Now I can't remember if the fleas were supposed to be attracted to it or if you'd just see them easier because of the white background. Either way, I saw tons when I did that when I couldn't find them before!