I found an amazing resource that shows the muscle you are stretching. These illustrations were created by Vicky Timón, a yoga expert and author of “Encyclopedia of Pilates Exercises,” and James Kilgallon, CSCS, creator of Mazlo’s Body Maintenance Program.
So many patients try to do the right thing and stretch. Unfortunately, many of them have been taught the wrong stretch for the muscle they want. Or they are in the right area but are doing the stretch wrong. For example, when you do a hamstring stretch your knee should always be a little bent. This moves the stretch from the pull on the muscle's tendon insertion and up into the muscle belly.
These pictures really help you zoom in on exactly which stretch does what.
Also remember, a stretch should be pain-free and you need to hold it for at least 30 seconds. You should also stretch after exercise, not before.
This pectoral stretch (29) has to be the one I give the most often, it helps reverse "desk posture" where the shoulders are hunched forward. I find this to be the cause of many instances of the upper back and neck pain, as well as contribute to headaches, RSI and carpal tunnel syndrome.