According to Osho, you know it is true witnessing when nothing remains to witness - only silent awareness without objects. Anything you can observe (even the thought, "this is pretense") belongs to mind; simply witness it and do not identify. Witnessing itself cannot be witnessed. Persist in disidentifying - hand, feelings, thoughts - using the simple test: the object of my knowing is not me. When all objects fall away, the witness alone remains.
If you can see it or think about it, it isn’t the real watcher; keep watching everything without grabbing it until nothing is left to watch and only quiet watching remains.