Listen for a "popping" or "crackling" sound. If you experience difficulty walking or moving your foot after hearing a "pop" or "crackle" sound, and did not strike your leg on something hard, then you may have torn your Achilles tendon. The popping may or may not be accompanied by pain for several reasons; your endorphins may be masking the pain, your tear is relatively small, or your nervous system has not yet detected the damage and sent a message to your brain indicating pain.
Test whether you can stand or walk on the affected leg. If after hearing the "pop" you suddenly can not stand or walk on that foot, there is a very good possibility you have torn your Achilles tendon. Be careful to put weight on the affected area slowly and stop once pain is experienced to prevent further damage to your Achilles tendon.
Check for swelling of the tendon. When injured, your body sends a multitude of fluids to the affected area to protect it and to prevent movement which might aggravate the injury. Swelling, with or without immediate pain, inability to stand, and a "pop" sound are sure signs of a tendon that has torn to some degree.
Lift the toes
Test whether you can raise your toes. If you can not raise your toes upward, then you have definitely damaged your Achilles tendon. In fact, in this case, you may have completely torn, or ruptured, your Achilles tendon and should seek medical attention immediately.