You have a patient with a limited range of motion in pain. What clues are there to diagnose an occlusal muscle issue versus an anterior displaced disc?
Taking a good history (the seven questions on the Dawson occlusal TMJ screening) is a great place to start.
After you measure the range of motion, brace your fingers on the incisal edge of the upper and lower teeth for 30-60 seconds. If there's a change-- if there's an increase in range of motion, that's called the soft end feel, which means it's probably muscle. If the measurement stays the same, then it's called the hard end feel, which means the disc is displaced anteriorly and getting in the way of range of motion.
Another great tool is a simple tongue blade or sugar stirrer. It's a great tool. You bite on the tongue blade, if there's pain on the opposite side, on the contralateral side, it's probably disc. Because the disc needs to rotate towards the side that you're biting on. If the pain is on the ipsilateral side, the same side that you're biting on, it's probably muscle.
These are just clues and like CSI, you need to analyze the entire complete examination before making a diagnosis.