How to differentially diagnose continuing sciatic pain given this existing information?

Question

Pains along the sciatic nerve apparently have numerous possible causes (18 of them according to this article).

Now, given the following facts, what would be appropriate differential-diagnostic measures/procedures/tests to determine the exact cause (or combination of causes) of such pains?

Relevant qualifying facts:

• Adult, but not elderly, male subject.
• Pain manifests only on one side of the body (the left side).
• Pain centered around the gluteal muscles (Piriformis?), occasionally shooting upwards or downwards.
• Pain intensity varies with time and posture; usually low intensity.
• Symptoms persist for over a year (but do not worsen).
• No relevant impact trauma.
• Regime of relevant physical exercise has little (and no lasting) effect.

Notes:

• I've tried to follow the guidelines in this answer and keep this question on-topic; if you believe it isn't, please comment and help me edit it accordingly.
• If you believe the cause(s) are obvious based merely on the facts above, an explanation why that is would be a valid answer of course.

Answer 1

DIFFERENTIAL DIAGNOSIS

Differential diagnosis in an adult man with a unilateral pain in the gluteal area lasting for over a year and not worsening, without a history of trauma, infection or inflammatory arthritis or other conditions (an incomplete list):

• Degenerative disc disease (DDD) with bulging or herniating disc(s) pressing upon the spinal nerve root(s) in the lower back (lumbosacral radiculopathy)
  • Often runs in families; can be caused by acute or repetitive injury
  • Pain triggers: coughing, bending and certain other moves
  • Pain relievers: walking, lying down (not always)
  • Other common symptoms: tingling, numbness, leg weakness
  • In most cases heals/improves on its own in weeks/months
• Hip conditions with pain radiating to the gluteal area
• Piriformis syndrome
  • Caused by repetitive running, climbing stairs, sitting
  • Pain is typically worse during sitting ("sitting intolerance")
• Ischial bursitis
  • Caused by prolonged sitting
  • Pain worse during sitting
• Sacroiliac joint dysfunction (sacroiliitis)
  • Pain in the hip and buttock
  • Worse during prolonged standing or running
• Spinal tumor
  • Pain can persist during the night
• Postherpetic neuralgia - inflammation of the nerve (neuritis) due to Herpes zoster infection
  • Typically (but not always) preceded by burning rash
  • Constant burning pain lasting for months/years

DIAGNOSTIC PROCESS

1) History

A doctor will likely ask detailed questions about the pain location, its triggers and relievers (coughing, sitting, walking, lying down).

2) Physical examination (examples of tests)

• Straight leg raise (Lasegue) test: pain triggered by the leg raise speaks for a pinched nerve in the lower back (lumbosacral radiculopathy)
• Provocative tests for piriformis (deep gluteal) syndrome

3) Investigations

• Blood tests to exclude inflammatory conditions (specific antibodies in ankylosing spondylitis and rheumatoid arthritis, sedimentation rate, C-reactive protein...)
• X-ray is often the first imaging investigation, which can detect broken vertebra, decreased space between vertebra (suggesting degenerative disc disease) and gross changes (spurs, deformities) due to inflammation
• CT or MRI of the lower spine or the hip is done when a problem with the discs or hip joint is suspected.
• There is no specific investigation for piriformis syndrome

NOTE: This is not a complete differential diagnosis of sciatica-like pain; it is a list of causes adjusted to the "case." The list of 18 causes linked from the question is grossly repetitive.