"The writing is on the wall" when you can see signs that there is going to be a change. Example: "At that point, the writing was on the wall; we all knew the company was going out of business." The signs pointing to the coming change are "the writing on the wall"; they are clear and visible to you if you look for them. Example: "Some people chose not to see it, but the writing was on the wall; the government had to change." Another example: "Can't you see the writing on the wall?"