Sciatica is mainly diagnosed by history taking and physical examination. By definition patients mention radiating pain in the leg. They may be asked to report the distributing of the pain and whether it radiates below the knee and drawings may be used to evaluate the distribution. Sciatica is characterized by radiating pain that follows a dermatomal pattern. Patients may also report sensory symptoms. 
Physical examination largely depends on neurological testing. The most applied investigation is the straight leg raise (SLR). 
Don’t forget to check for red flag conditions, such as malignancies, osteoporotic fractures, radiculitis, and cauda equine syndrome. Take a history to determine localization, severity, loss of strength, sensibility disorders, duration, course, influence of coughing, rest, or movement, and consequences for daily activities. In cases with a dermatomal pattern, or positive result on straight leg raising test, or loss of strength or sensibility disorders test the reflexes (Achilles or knee tendon), sensibility of lateral and medial sides of feet and toes, strength of big toe during extension, walking on toes and heel (left-right differences), and straight leg raise. 
The most common differential diagnoses for sciatica are lumbar disk disease (herniated disk), degenerative changes, lumbosacral strain, neoplasms of the spine, and piriformis syndrome. Less common are infection, ankylosing spondylitis, and pelvic masses. 
In the case of a disc herniation, the person will complain of increased pain when sitting or standing with a forward flexed posture (bending). This will cause more leakage of the nucleus pulposis, thus aggravating the compression on the nerve. Conversely, bending backwards may cause the pain to diminish, because the nucleus pulposis will go back to the spine, taking the pressure off the nerve. 
When a patient presents with low back pain that radiates into the buttock, down the posterolateral aspect of the leg, and sometimes to the foot, and then complains of episodes of paresthesia, the physical therapist first should attempt to rule out herniated intervertebral (lumbar) disk. Lumbar herniated disk is more prevalent in men than women, between 25 and 50 years old. Symptoms include sudden onset, unilateral pain that radiates to the buttock down the posterolateral aspect of the leg and sometimes into the foot, and possible paresthesias and/or neuromuscular deficits in the dermatome regions associated with disk herniation. More than 90% are between L4-L5 or L5-S1. Aggravating factors are coughing, sneezing, or the Valsalva maneuver. Alleviating factors are standing and lying on the unaffected side with both knees flexed. 
The appropriate diagnostic tests to rule out this diagnostic is a physical examination focusing on location of pain, muscular strength reflexes, and sensations in both lower limbs) and straight leg raise test. One usually sees limitation in flexion and extension of the spine, lost ankle jerk on the affected side, possible muscle weakness and/or sensory loss in the lateral aspect of the tight and calf, and a positive straight leg raise test. A CT scan can show evidence of disk prolapsed. 
In case of degenerative changes in the lumbar spine, most common on aged people, the narrowing of the spinal canals causes direct pressure on the nerve leading to radiating pain in the buttocks and the back of the thigh and leg. These symptoms may become less intense when bending forward which opens the canal, taking pressure of the nerve root. By the contrary, bending backwards will narrow the space, and compressing even more the nerve root, increasing the symptoms. 
Lumbosacral strain is another differential diagnosis to consider. It is equally prevalent in men and women, and generally occurs in those 25 to 40 years old who are obese and/or physically unfit. A strain usually occurs after lifting, twisting, or trauma. The symptoms include immediate or delayed onset of pain after a traumatic event to the lower back region and unilateral pain that radiates across the lower back with occasional radiation into buttock and rarely to lower extremity. The pain is usually described as dull and persistent and associated with stiffness, and patients have difficulty locating a precise point of maximum pain. Aggravating factors include standing, bending, and most motion (lifting, twisting), worsening as the day progresses. Alleviating factors are rest and sitting. Neurological exam is negative, and there may be some limitation of motion anteriorly and posteriorly and some increased lumbar lordosis. Straight leg raise, if positive, elicits pain in the back. 
Lumbar stenosis, another differential diagnosis, it is generally seen in patients who are older (more than 60 years). It usually presents with gradual onset of pain in the lower back that may radiate into buttocks and legs with exercise, prolonged standing, or walking up stairs. These patients usually walked in a stooped position (flexion of spine) and are forced to rest after walking one or two blocks. Rest, sitting and lying on their back on the floor alleviates the pain. This condition cannot be diagnosed only with the x-ray showing space narrowing and spurring because by the age of 50, 80% to 95% show stenosis. 
A final diagnosis to be considered is that of piriformis syndrome. The symptoms of piriformis symptoms are sudden, unilateral buttock pain that may (or not) radiate down the thigh and lower leg. For the assessment of the piriformis syndrome, the physical therapist can perform the adduction/internal rotation/flexion test (AIF). 
The adduction/internal rotation/flexion test (AIF) test involves having the patient lie on his/her unaffected side, bend the knee of the affected leg to a 90-degree angle, and catching the foot behind the calf of the healthy leg, swing the affected leg over the healthy one until the knee touches the examining table. This test tightens the piriformis muscles' grip on the sciatic nerve and buttock and/or thigh pain on the affected side indicates a positive sign. 
Physical therapy treatment should be the first treatment to be considered in case of sciatic, primarily aimed at reducing pain and returning the patient to his normal daily activities, by reducing the pressure on the nerve root.
A complete physical therapy treatment should contemplate accessory and active mobilization of the lumbar vertebras, to free up the joint motion, thus allowing the nerve root and the sciatic nerve to move more freely. A massage to the muscles in the low back, and buttock regions, paying especially attention to the piriformis muscle, for free up soft tissue motion, helps releasing any nerve entrapment that can and possibly will exist. Also, initiating a good flexibility and strength for the muscles in the lumbar area and in the lower extremities would help in the prevention of repetitive sciatica injuries. If the patient is heavy weighted, probably reducing the body weight is a good idea for preventing future complications.
If driving long periods with the foot on the accelerator and/or sitting with a heavy wallet in the back pocket contributes for sciatica, taking breaks when driving and removing the wallet should be a good solution.
If running is the problem, changing sneakers every 250-400 miles is a good idea, because sneakers lose 40% of its shock absorption capabilities.
If biking is the problem, then a proper bike fitting may help prevent this injury. Be sure to consider bike size, seat height, and handle-bar height/angle.
 Koes, B., W., van Tulder, M., W. et Peul, W. C. (2007). Diagnosis and treatment of sciatica. BMJ; 334: 1313-1317
 Douglas, S (1997). Sciatic pain and piriformis syndrome. The Nurse Practicioner; 22
 Dubin, J.C. (2002). Injury Management Update