The best way to diagnose osteoporosis or osteopenia is with a DEXA Scan. That's something your orthopedic surgeon or your family practice physician can order for you, and basically, a DEXA Scan is the most reliable way with the least amount of radiation to measure your bone density. It takes different images of your spine, and your hip and compares that against the bone density in a normal 25 year old. It gives you something called a T-score. Your T-score will put you on that spectrum of the osteoporosis or osteopenia. If you get a T-score between 1 and a 2.5 which measures the standard deviation of your bone density compared to that normal person, that puts you in that range of osteopenia. If it's anything less than 2.5, a standard deviation below a normal bone density, then you're in that category of osteoporosis and that's when you should really be started on some sort of treatment, or therapy for it.
