The Mound-
Fire ant mounds may look similar to regular ant mounds at first glance, but there are some ways they differentiate. One of which is whether or not there is an entry hole at the top of the mound. If there's no hole, chances are it's from a fire ant colony. Fire ants enter and exit via underground pathways, not from the top of the mound. These mounds are usually just a few inches high, but they can be over a foot high if given enough time. You can still have fire ants even if there is no mound.
The Sting/Bite-
Ant bites or stings, regardless of species, are unpleasant. However, fire ant bites can be downright painful. Their "bites" (actually stings) cause a burning sensation and leave a blister filled with fluid. Some people are allergic to fire ant venom and their reactions can be far more severe.
The Appearance-
Fire ants have distinct physical differences than regular ants. They have a dark reddish color which other ants don't have. They also vary greatly in size, unlike other ants that are more often than not the same size.