Normal medical imaging (e.g. an MRI) is not effective in diagnosing BPPV, because it does not show the crystals that have moved into the semi-circular canals. However, when someone with BPPV has their head moved into a position that makes the dislodged crystals move within a canal, the error signals cause the eyes to move in a very specific pattern, called "nystagmus".

The relationship between the inner ears and the eye muscles are what normally allow us to stay focused on our environment while the head is moving. Since the dislodged crystals make the brain think a person is moving when they’re not, it mistakenly causes the eyes to move, which makes it look like the room is spinning. The eye movement is the clue that something must be happening mechanically to move the fluid in the inner ear canals when it shouldn’t be. 

The nystagmus will have different characteristics that allow a trained practitioner to identify which ear the displaced crystals are in, and which canal(s) they have moved into. Tests like the Dix-Hallpike or Roll Tests involve moving the head into specific orientations, which allow gravity to move the dislodged crystals and trigger the vertigo while the practitioner watches for the tell-tale eye movements, or nystagmus.

There are two types of BPPV: one where the loose crystals can move freely in the fluid of the canal (canalithiasis), and, more rarely, one where the crystals are thought to be ‘hung up’ on the bundle of nerves that sense the fluid movement (cupulolithiasis). With canalithiasis, it takes less than a minute for the crystals to stop moving after a particular change in head position has triggered a spin. Once the crystals stop moving, the fluid movement settles and the nystagmus and vertigo stop. With cupulolithiasis, the crystals stuck on the bundle of sensory nerves will make the nystagmus and vertigo last longer, until the head is moved out of the offending position. It is important to make this distinction, as the treatment is different for each variant.