You'll Know It When It Happens

 is a quest in Fallout: New Vegas.

Detailed walkthrough
There are three different assassination attempts: the first is a sniper dressed as an NCR Ranger who will appear atop the guard tower behind the stage where the president gives his speech. The second is a bomb placed on the Vertibird by an engineer. The third is a Leigon soldier disguised as an engineer who will run up to the President and try to kill him with a knife.

If you wish to complete the optional security sweep, there are two things to find: a bloodstain in the Supply Closet (requires 6 perception), and recent unauthorized activity on the computer terminal in the Visitor's Center (requires 50 science). While in the Supply Closet, you should take the NCR Engineer Jumpsuit from the locker. You can now tell Ranger Grant you're ready.

If you received this quest from Yes Man, you will not be able to access the top of the Visitor's Center where the Presiden't Vertibird lands unless you are disguised as an NCR member (this is where the NCR Engineer's Jumpsuit from the Supply Closet comes in handy), however if you can access the landing area for the Vertibird, and you have 50 explosives or repair skill, you can disable the bomb. Wait until the Vertibird has landed and the President left it, then "Activate" it and you will receive a message asking you to check for the bomb.

Whether you have been able to defuse the bomb or not, you still need to stop the assassin at the guard tower. You will see him throw the guard off of the guard tower and take his place (it's a very noticeable scripted event that will occur unless you're waiting atop the tower when the assassin appears, in which case he will attack both you and the real sniper immediately). Kill the assassin. It is possible to both defuse the bomb and reach the top of the tower in time, but it is not a foolproof method. The easier option is to defuse the bomb, then run and wait near the large anti-aircraft gun (though don't go behind the stage, as everybody will turn hostile if you do; not known whether this is a bug or not (confirmed Xbox 360)). When you see the Ranger thrown off the side of the guard tower, you know it is time to take out the assassin using your weapon of choice. If you are a good enough sniper, the assassin can be killed from a position atop the same building the Vertibird lands on.

An alternate method to completing this quest is to forgo defusing the bomb (for example if going atop the Visitor's Center will turn the NCR hostile, or if you lack the sufficient repair or explosives skill), and wait atop the guard tower where the assassin will appear. Kill the assassin when he appears from a hatch in the corner, and then use the radio on the table to contact Ranger Grant. Letting him know of the attempt will cause him to cut short the President's speech and sweep the area again, causing him to find the bomb and defuse it. After you have radioed to Grant, go down by the stage, and watch for the (Legion) engineer who will run up to the President, and kill him if he does. The President will then escape safely.

(Optional)
Investigate the Area for Clues. -Examine (Perception 6) a blood stain in the Supply Closet in the Hoover Dam Visitor Center. -Hack computer in Visitor Center to discover an unauthorized access ID.

Behind the scenes

 * After killing the Legion sniper on the guard post and using the radio to report to Grant, one of the dialog options ("Everything's fine up here, thank you. How are you?") Is a reference to Han Solo from Star Wars episode IV: A New Hope when he and Luke Skywalker are saving Princess Leia from the Death Star.