Would there be signals of a 51% attack or could this occur with no warning?
I suspect that the most likely attack would be a sudden large scale blockchain reorganization going back many weeks or months. You wouldn't know it was being prepared, but the results would be immediately visible once the competing blockchain is broadcast.
As far as I am concerned, most clients implemented checkpointing, so at most you can have a reorganization going back to one/two days ago, correct me if I am wrong.