Diagnosis...

A special type of Xray can assess the density of your bones. Known as Dual energy Xray Absorptiometry (DXA, or DEXA) this type of Xray shows the doctors if you have abnormal bone density. The assessment will give you what is known as your T score.

According to the World Heath Organisation (WHO):

If your T score is 1.0 or greater, this is 'normal' bone density.
T score between –1.0 and –2.5 is 'low bone mass' or 'osteopenia'.
T score of –2.5 or lower is diagnosed as osteoporosis.

A 'low trauma' or osteoporotic fracture, defined as one that occurs as a result of a fall from a standing height, is also diagnostic of osteoporosis regardless of the T-score.

In order to differentiate between "primary" (post-menopausal, regardless of age) and "secondary" osteoporosis, blood tests and X-rays are usually done to rule out cancer with bone secondaries, multiple myeloma, Cushing's disease and some other conditions.