First things first, let’s show you where the peroneus longer and brevis are (peronei tendons). See the picture below. The two tendons come down the outside of your calf and ankle and are tucked behind the big ankle bone that sticks out on the outside of your ankle. As you can see, they then travel down in an L shap and plug into your foot bones. On the other hand, the ligament involved in an ankle sprain is the anterior talofibular ligament. It is right in front and below the big ankle bone.
So, if the pain is not in front of the outside ankle bone, it can’t be a peronei tendonitis.
Now, the way in which you hurt your ankle also tells you what’s going on.
If you hurt your ankle by rolling it, most likely you have a sprain AND the pain is right in front and below the big outside ankle bone, you most likely have a sprain. It is possible to strain the peronei with an ankle roll, but much less likely. When rolling the ankle affects the peronei and not the ligaments, the pain is usually where it plugs into the bone of the foot (see pic above). This will not be right next to the big ankle bone, but in the tip of the bone of the foot.
Most of the time you have a proper peronei tendonitis is when you didn’t hurt yourself instantly, but started developing pain for no known reason. And the pain will be behind the ankle bone and sometimes even up the outside of the calf.
How you treat the injury depends on what the injury is. So make sure to have your ankle region pain properly diagnosed!