Cats are great groomers and clean freaks. Their tongues are really rough specifically so they groom themselves with it. Because they are so good at grooming you won't often see a lot of fleas on them. There are a few things though that can give it away.
1. Look under their chin.
You'll often see live fleas here or flea dirt. It's the one place on their body that they can't reach to groom with their tongue or their paws. The fleas will often hide out here so lift up that chin and part the hair