Fleas move quickly, making them difficult to spot. Place your cat on a sheet of white paper and comb them meticulously - a fine-toothed comb may show up a couple of fleas and 'flea dirt' - flea droppings, can often be found too.
If your cat is allergic to fleas, you might notice that they develop itchy skin, hair loss and small scabs around the tail or neck. If this is the case, it is best to visit your vet.