Check the muscle for swelling. Does it look more swollen than the healthy one? If so, there is a good change you have a torn or strained muscle.
Check for "knots" in your calf. A muscle spasm, commonly called a knot, is caused when healthy muscles try to compensate for the disabled one. Also, your body will send chemical signals to the affected area, causing the surrounding tissue to try to protect the torn muscle. Both knots and swelling are an almost-certain sign of a torn calf muscle.
Test your leg by standing or walking. Favoring is when you take shorter steps, put less weight on your leg, or otherwise abbreviate its natural movement. If you have swelling, knotting and are favoring your calf, you are definitely suffering from a torn muscle and should be treated.