Symptoms will of course indicate the side of an HNP by the side of leg pain.  A large HNP might cause pain in both legs. A midline HNP might cause back pain only.  The level of the HNP can be inferred from the neurological testing, If a nerve root has been impaired, then often weakness, sensory loss or reflex changes can be found.  

Weakness of big toe, decreased sensation over the big toe and/or a decreased or absent posterior tibial reflex usually indicate HNP at the L4/5 level getting the L5 nerve root  If sensory decrease is over the little toe or the ankle jerk reflex is decreased or absent, then the HNP is most likely at the L5/S1 level.

An L3/4 HNP occurs less often; it may produce a decreased or absent knee jerk reflex and decreased sensation just below the knee.  If these neurological findings don’t match the imaging location of a HNP, it might be advisable to repeat the neurological exam and consider getting another imaging study, especially if your initial imaging is more than one year old.