It's really obvious once the new chain is broadcasted. "Honest" miners switch over to mine on the deepest chain as soon as they become aware of it. It's very rare to even see a 2-block conflict. When you see a large N-block conflict then it's obvious there's a 51% attack going on.

But isn't the point of the whole thing that you do not broadcast it until after you are done? So yes, it's obvious in hindsight, but the damage will be done.

The question was how do you detect it. You obviously can't detect it until it's broadcasted because until then it's all happening in secret.