House flies are attracted to food sources, water, and light. Swarms of flies are common around decomposing waste, where they can get both food and moisture. Look for fly larvae (maggots) in waste and rotting food. The small white worms eat hardily for several days before pupating into their adult form.
When house flies are nearby, you may hear the telltale buzzing noise of their wings beating together. Listen for the sound around garbage, dumpsters, drainpipes, and light fixtures.