How do I know if I'm experiencing Achilles and / or peroneal tendonitis?

A good exam or MRI. A thorough history and physical by a podiatrist or qualified physician is the best way to determine if you have achilles or peroneal tendonitis. An MRI will give a more difinitive answer, if needed.
Area of Pain. In achillis tendonitis the pain is in the back of the heel usuall near the area where the achillis tendon inserts into the calcaneous or heel bone. Peronial tendonitis is on the lateral side (outside) of the foot. The pain can feel like it is from the lateral side of the ankle, and can radiate up from the ankle, or down the lateral side of the foot.
Location of pain. Achilles runs in back of the peroneal which courses the outer or lateral aspect of the ankle.