A herniated (slipped) disc in the neck (cervical) spine causes pain in the neck, between the shoulder blades and often radiates down one arm. Placing a hand on the affected side on top of the head often relieves the pain. Applying pressure on top of the head often aggravates the pain in the arm (Spurling's test - video, 1 min).
A herniated disc in the lower back (lumbar) spine causes pain in the lower back, which often radiates down across the buttocks and into the thigh. Raising an extended leg to a certain level (straight leg raising test, video, 20 sec) triggers pain in the buttocks. The pain is worse during sitting or lying and relieved by walking.
A herniated disc in the chest (thoracic) spine is very rare.
Muscular pain is by far the most common cause of back pain, either due to bad posture, repeating chronic injuries or acute injury.
A herniated disc can be detected by an MRI.