If your dog scratches itself excessively, bites its rear, tail end or thighs, loses hair, has skin lesions and/or red bumps and, on top of that, you find flea excrement (tiny black fecal matter; if you add water to it and smear it using toilet paper it gets red in colour) - there's a pretty good chance that your dog has been infested with fleas. Don't hesitate to act; flea infestation may have a bad influence on the health of your pet. Apart from the obvious discomfort, many animals suffer from FAD, i. e., Flea Allergic Dermatitis, which can be tough on your dog.
How do you get about treating your pet, then? Well, first and foremost, remember that chemicals are the last resort.