How do they test for low bone density?

Bone densitometer. An instrument called a bone densitometer is utilizes a very small dose of radiation to measure bone density in the spine, hip and forearm.
DXA. The standard imaging modality to assess your bone density is the dual-energy x-ray absorptiometry or dxa (formerly known as dexa). Do not confuse it with bone scan however. Women over the age of 65 should get a dxa scan (or sooner if risk factors are present).