Give yourself a little hug by wrapping your arms around you. Where your fingers lie on the shoulder blade is more than likely going to be the correct area (The infraspinatus). Dig your fingers in a little and have a feel around. If there is an issue with this muscle then you will probably feel it.
Then using a tennis ball or trigger point ball, place on the spot that you found and back away into a wall so that you can put more pressure on. When resting on an uncomfortable area, take 3-5 deep breaths. Find a couple of different spots of discomfort and repeat .
You may also notice referred pain in the front of the shoulder which is not uncommon in this muscle. Complete 1-2 times a day especially before activity and after periods of inactivity.