Watch the behaviour first
Excessive scratching, biting at the skin, licking, or chewing, especially around the base of the tail, groin, belly, and behind the ears, is often the first sign owners notice. Some dogs seem restless or unsettled, repeatedly getting up to scratch or reposition. Not every dog reacts the same way; some tolerate a light flea burden with barely any visible discomfort, while others, particularly dogs with flea allergy sensitivities, can have a strong reaction to even a single bite.
What to look for on the skin and coat
Part the fur, particularly around the tail base, and look for small, fast-moving dark brown insects roughly the size of a sesame seed. You may not spot live fleas easily since they move quickly and avoid light, so "flea dirt" is often a more reliable clue, small dark specks that look like ground pepper scattered through the coat or on the skin. A simple test is to comb some onto a damp white paper towel or tissue; flea dirt is digested blood and will turn a reddish-brown colour when wet, whereas ordinary dirt won't.
Skin changes that suggest a flea problem
Look for small red bumps, scabs, hair loss, or redness, particularly around the tail base, groin, and inner thighs. In dogs with flea allergy dermatitis, even a small number of bites can trigger widespread itching, hot spots, and hair loss well beyond the actual bite sites, because the reaction is to flea saliva rather than the bite itself. Persistent skin irritation that doesn't improve, or that seems worse than the number of fleas you can find would explain, is worth a vet visit.
Other things fleas can cause
Fleas can also affect general wellbeing indirectly. A heavy flea burden, especially in puppies or small dogs, can occasionally contribute to anaemia from blood loss, and fleas can transmit tapeworm if your dog swallows an infected flea while grooming, seeing small rice-like segments near the tail or in stool is a related sign worth mentioning to your vet.
What to do if you suspect fleas
Check your dog with a fine-toothed flea comb over a light surface, focusing on the areas mentioned above. If you find live fleas or flea dirt, talk to your vet about an appropriate treatment for your dog's age and weight, and plan to treat your home and yard as well, since fleas found on your dog usually mean the environment is affected too. Checking other pets in the household at the same time is worth doing, since fleas move readily between animals and an untreated cat or second dog can keep reintroducing the problem, undoing progress on the dog you've already treated.