How do I know if I ruptured my Achilles tendon?

Gap and weakness. A gap that you can feel where your achilles should be in the back of your leg/ankle (compare with normal side), and weakness during foot pushoff are signs of a possible rupture. Normally people describe being shot in the back of the leg with an achilles rupture. Further clinical evaluation by an orthopedic surgeon will confirm your suspicion. An MRI is not usually required.
Achilles rupture. Most common things i hear from patients is intense pain " like i got shot in the back of my heel" . It is a serious problem and seek attention. An MRI will be ordered to make sure it's a rupture.
Symptoms of rupture. Symptoms of achilles’ tendon rupture include: weakness, and lack of push-off strength, instability and difficulty to walk, swelling around the area, more apparent when compared to the other leg, sometimes pain around the area of rupture, sometimes there is a noticeable dell or depression in the area where there used to be a heel cord. http://www.prweb.com/releases/2013/4/prweb10672931.htm.
Depends. If you feel a gap or visibly see one foot hanging down more than the other or when you squeeze your calf muscle one foot flexes but the other doesn't move then you have a rupture. Tears would need medical eval as this is best for an expert analysis vs you testing it.