How does one confirm a 51% attack and how to repair the damage?

[CoinFascination]

Wondering if there are services that watch for this eventuality?
Or perhaps some code that checks it.
But after the fact, when the attack has happened, how does one decide on a rollback?
To what does one rollback too? And what does that mean exactly?
Telling the exchange to modify the blockchain or giving them a bootstrap?