Anyone know where this phrase came from? (Or, “I should have seen the writing on the wall”?) What wall? To what writing does it refer? I keep picturing that scene in Monty Python’s “Life of Brian” where a citizen writes something like “Romans go home!” on a wall. But, he doesn’t write it in proper Latin, so a Roman Guard makes him write it 100x! 
Wiki to the rescue.
It’s a Biblical reference.
The writing on the wall at Belshazzar’s feast in the book of Daniel.
As others have already noted.
I’ve always liked the bit we sing in Walton’s oratorio Belshazzar’s Feast: “Thou art weighed in the balance, and found wanting”.
Recent thread, which never did get an answer to the question being asked.
It is from the Book of Daniel. Where in the Palace of the king of Babylon a hand writes upon the wall “Mene, Mene, Tekel, Upharshin.” Since the king’s interpreters could not figure out what it meant (it could not be anything good), Daniel is called in to translate.
He tells the king that his kingdom is on the way out, he has been judged and found lacking, and his kingdom is going to be divided between two kingdoms.
Brian wrote: ‘Romanes Eunt Domus’, or ‘People called Romanes they go the house’