Sometimes you may see a live flea on your dog- but they are very fast so can be hard to spot. They often prefer to live on the dog's back and near its tail, so these are places to take extra care to check.
Often, we do not see the actual flea itself but see flea dirt which is excreted dog's blood. It looks like black dirt in the fur, but if rubbed with a damp bit of cotton wool you can see the redness of the blood, which helps to determine it from simply dirt from the garden.
Sometimes the first thing that you can notice is the effects of fleas on your dog- they might be rubbing, scratching or chewing their skin. Sometimes the skin looks inflamed (red) and in severe cases the skin can be traumatised and become scabby and even bald.