Fleas have 4 stages, 2 of which - eggs and pupae - no insecticide can kill.
The most common sign of a flea problem is their irritating bites. Fleas only jump around 1-2 ft off the ground, therefore bites are usually found around the lower leg and ankles. However if fleas have infested a bed or sofa they will bite any part of the body.
Pet problems:
If your cat and dog are regularly itching and scratching, it is likely they have a problem. Check pet bedding for signs of the fleas. If you catch it early you may only have to treat the pet bedding rather than the whole house.
Sightings: If you have a problem you can often see them jumping about. These insects generally look black and are approximately 1mm in size.