Bone Mineral Density (BMD) Testing
One of the best ways to find out if you're losing bone is to have someone measure your height, and then check it every six months or so. If you start losing height, that's a sure sign that you're losing bone on your spine. I recommend that women at risk for osteoporosis get a bone mineral density measurement as they're going into menopause. That way you'll have a baseline with which to compare later bone density tests, to measure your progress.
The safest and most accurate ways to measure bone are with Photon Absorptiometry, and Dual Energy X-ray Absorbtiometry (DEXA), which is 96-98% accurate and uses very low-dose x-rays. I don't recommend CAT scans, as they use too high a level of X-rays. A newer technique for measuring bone loss is called "Urinary Excretion of Pyridinium," which measures a substance in the urine that can indicate rapid bone turnover rate.