So right now I'm doing the quest: "I Put a Spell on You" where it can be obtained at Camp McCarran (NCR's Main Base). I'm at the part of the quest where I'm checking the watch tower or what ever it is investigating the late-night break-ins. Can anybody tell me exactly what to do here? I honestly have no clue... I checked the terminal and assumed I'd have to catch the spy in the crime but had no luck.

Go in there at 1AM.

It didn't work
It's the tower with the small room, HAM radio, and a wall radio right?
And am I looking for someone in there?

I've had this issue as well - just stood in there from 9PM to 5AM and nothing happened.

Maybe I did something wrong.. I'll try again later.

First, talk to the Old First Recon guy, the one with the unique cowboy repeater and he'll tell you he saw someone up the tower late at night, around 1AM.

Then DO NOT TALK to anyone else, just go wait for someone to enter at 1AM.

DO NOT DO NOT DO NOT talk to any NCR officers about it, it breaks the quest if you talk to the mole about it and events won't trigger the way they're supposed to, if you end up screwing it up the leak will just sit there inside the tower and you cannot overhear.

Assuming you do like I said above you want to enter at 1AM just after the spy, overhear what he says, do not engage him, run your ass to the monorail and disarm the bomb (35 explosives and like 50 science, don't quote me on it), THEN report to Colonel Hsu.

The key is to not engage the mole and not to let on to anyone that you know there is a mole at the tower at 1. Then sneak in, sit there, leave, disarm bomb, talk to Hsu.

If you are in the tower, the guy won't come (iow, he must somehow know you're there). You can find out the time (which is, 0100 hours) by talking to the First Recon member that was mangled by the legion.

To do this, you have to be in the vicinity of the tower, starting at 2330, and then wait - but do not use the 'wait' command. Just squat there, use your binoculars, or your scopes, and keep an eye out.

Eventually, you'll see someone make their way towards the tower. Now before going in, make sure that you're in sneak mode, and don't rush up. Just go in, and wait 15 seconds. You might have to rush somewhere else first.

On my first playthrough, I used 'wait' comman, and entered with sneak at 1:54 and it worked. In fact, the spy was sneaking right in front of me on the stairs on his way out, and then started his script to talk to his boss. After the quest said to defuse bomb, I went straight there and defused without incident. I think I had told my companions to wait outside (so they dont follow me in)

On my 2nd playthrough, I just could not get it to work! (This is after the game got its first patch). Anyways I don't know what makes it work or not ( I talked to a zillion people after I talked to the La Lounge Carabine guy who tells you 1am so I thought I was sunk), BUT after reading this thread I got an idea.

I ran far away (30sec hike) and dropped my companions there. Then I entered sneak mode, then 'wait'ed until '1:40', then snuck all the way to the door (by the time I got to the door it was almost 2am). It worked the first time! The mole was there. I wait until the quest updated to my objective being 'disarm bomb' (dont leave too early!)

Again, I dont have the patience to keep retying this black-box testing style, but hopefully it works for some of you.