We are working together on a project and he may try to hide it, but I know he's almost always just either watching videos on YouTube or browsing places like Reddit. If he wasn't missing deadlines I wouldn't raise an eyebrow but as of right now he isn't very productive.
First of all, every case is different. If this is something that is not impacting your work, I would stay out of it. But I do think it is appropriate to bring it up to the person's manager or your manager. If you can't do that for political reasons, then maybe you need to find a place to work where the lines of communication are more open.
If this is your employee, you need to bring it up to them right away. Don't wait for review time, you need to be giving feedback early and often for this kind of behavior. And you also need to record the time and place if you anticipate having to formally discipline the employee in the future. When you talk to the employee, don't just say, "slacking off." Be specific about the times, places, and kinds of behavior you are observing and what your expectations are in the future.
Also, approach the subject with an open mind. Before starting to talk about the behavior, ask the employee what is going on with them, are they happy/challenged in their job. You might be surprised what you hear back.
"Before starting to talk about the behavior, ask the employee what is going on with them, are they happy/challenged in their job. You might be surprised what you hear back."
I totally agree. The most common case I see is that if someone is slacking, procrastinating, or otherwise spending a bunch of time on entertainment types of sites during work hours, it's because they don't feel like there's a clear next goal. They can be demotivated, under-challenged, or simply slightly depressed with how things are moving forward. It's easy to slip into this unproductive mode as a way of avoiding the pain of the present lull.