Hi! I think that bilateral VN is much more unusual...I think maybe one side is hit and then the other side is hit later???, but again very rare.
You will need to have inner ear testing: VNG test, including a caloric test, and possibly a rotary chair test, which I believe is more useful to determine bilateral damage. Best doc to see for this is a neurotologist.
I just finished up with all of the testing and I think they look at a combination of the testing and the symptoms...
I know the caloric test had a lot to due with the diagnosis of bilateral, but they also talked about feeling "off balance" rather than a spinning vertigo, problems worsening without visual cues (eyes closed or in the dark) and oscipllopsia as indications. I didn't have to do the rotary chair test.