Perfectly cooked shrimp will turn from translucent to opaque pink in color.  As they cook, they will form into a nice loose "C" shape; however, if they form into a tight "O" shape this is an indication that they're overcooked. For a sure-fire way to tell if shrimp are cooked properly, use a meat thermometer to check the internal temperature.  This is done by inserting the meat thermometer into the thickest part of the shrimp.  When the temperature reaches 140 degrees F, they're done.