The Hoover Dam checkpoint building is a makeshift shack built on top of Hoover Dam. It is inaccessible to the Courier until the final quest. The checkpoint building is scrappy, with lockers and metal boxes strewn about the room. There is nothing of value to take, unless the Followers of the Apocalypse have been convinced to help NCR, in which case there will be a medical supply kit in the shack. In the room, there is a ladder that leads to a watchtower that looks over Hoover Dam.
Inside the shack towards the south on a table is a terminal which grants information about the Dam's generators. There's also an option to run a diagnosis of the generators. If the Courier has a detonator in his inventory, it's possible to rig the generators to detonate them. Detonating them will show an EMP-like effect around the checkpoint, killing any Legion troops on the other side of the checkpoint during the NCR quest Eureka!.
The Hoover Dam checkpoint appears only in Fallout: New Vegas.