Check the muscle for pain. Pain can come on suddenly (acute), such as after lifting something heavy, or may be caused by repetitive motions and come on more slowly. Either way, pain is a certain sign that something is wrong.
Check the area for swelling. If the muscle is swollen, then fluids are rushing to the affected area and causing it to swell to protect it. If the area is swollen--especially if accompanied by pain--then it likely is a torn muscle.
Check to see if your body is "favoring" a muscle. Subconsciously, your body will try to prevent further injury by favoring--that is, treating gingerly--the affected area. For example, you may be favoring your right leg if you do not put your full weight on it or if you do not use your full range of motion.
Feel around the affected area for "knots" in your muscle. When a muscle is injured, your brain tells the other muscles surrounding it to spasm in order to protect the muscle. If you have a "knot" in your muscle, you are experiencing a spasm, which may indicate a torn muscle.