You'll Know It When It Happens

 is a quest in Fallout: New Vegas.

Detailed walkthrough
There are two 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.

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).

If you can access the landing area for the Vertibird, and you have 50 explosives or repair skill, you can disable the bomb. It is placed there by an engineer almost immediately after the Vertibird lands.

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). Kill the assassin, or let the NCR Rangers around you do it (though this method doesn't always work). 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. 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, and then the engineer who placed the bomb on the Vertibird. It is important to note that there are three engineers present, two of which are genuine engineers and not attempting to kill the President - killing either of these two engineers will result in the NCR becoming hostile. Finding the rogue, bomb-planting engineer is not always easy. Sometimes he will run up to the stage where the President gives his speech, sometimes he will remain atop the Visitor's Center where the Vertibird landed. However, as long as you kill the correct engineer, the bomb will not detonate and the assassination attempt will fail.

The radio atop the guard tower can be used to contact Ranger Grant after killing the assassin. Letting him know of the attempt will cause him to cut short the President's speech. Ranger Grant can also be informed of the defused bomb to cause him to cut the speech short, but if this is done you still need to watch out for the assassin atop the guard tower.

(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.