The ants emerge out aggressively when they are disturbed and sting. Their sting usually leaves a white pustule the next day.
Harvester ants are much larger and make large bare areas with a single entrance hole to the colony.
Leaf cutter ants are also much larger and do not have a distinctive built up mound, but do have many entrance holes over a very large area. Other small to medium sized ants that build small mounds will actually run away from disturbances and aren't fire ants. Native fire ants are less common in imported fire ant infested areas. Solenopsis geminata is the most common native fire ant species encountered. To the unaided eye, they are almost identical to red imported fire ants.