I found the B12 oils, which provide a systemic B12 dose equivalent to an injection, mainly helped my brain fog, but not other ME/CFS symptoms.
Before ME/CFS, I used to work as a computer programmer. But the brain fog of ME/CFS normally prevents me from doing any coding. However, I found that once I started taking B12 oils once weekly, my ability to program returned to an extent, and I was able to start some computer programming projects at home (just for my own amusement). So this is an objective demonstration that the B12 improved my brain fog.
I may not have noticed these cognitive benefits of B12 oils had it not been for my ability to program returning, and testing this by starting some coding projects. It's good to pit yourself against a difficult cognitive task like this, to see if your performance at the task increases with B12. That way you can objectively measure any improvement.