I think I injured my shoulder while bench pressing. How do I check where my injured muscle/area is?

Hard to say. If you have pain, there's a good chance the injury is where it hurts. You need to say what makes you feel you have an injury. Some are easily diagnosed on the surface by looking, palpating and moving the shoulder. Others require imaging such as x-ray or MRI to visualize the precise nature of the injury. But if there is not pain or loss of motion, do not worry.