My jaw hurts how do i know that my jaw is locked?

If truly "locked", you could not open fully or have full lateral excursions. Have a dental examination and assess whether this is indeed a TMJ issue. Preventative care can certainly lead you away from "jaw is locked".

Did you have a blow to the area or do you grind your teeth at night? Stress can cause you to grind your teeth or if you yawned to wide or opened too much than muscles are strained and pain will result .You may have a TMJ problem or need a bite plate. See an oral surgeon for a diagnosis.