In general, I'd say don't bother. Just do a bit of pre-recording testing to make sure your input levels are good. You're not going to want to tweak the input levels a bunch while you're recording, anyways. That would just be distracting to you and the other people that are recording.
But if you really want to, and you're on a Mac, check out "Line In" from Rogue Amoeba: http://rogueamoeba.com/freebies/ - I just tried it out, and it works great.
If you're on windows, this may work http://answers.microsoft.com/en-us/windows/forum/windows_vista-pictures/how-to-get-the-microphone-sound-through-speakers/1643d977-5dfb-4eca-9330-3b16370e7646 though I haven't tried it (don't have my windows machine ready to try it out). There are other options, as well, like software packages you can purchase.
One thing you need to know, though, is that you'll end up with a small delay between your actual voice and what you're hearing through your headset. It's not much of a delay, but it is noticeable. This can be confusing if you're not used to it. You might accidentally start to slow down or pause or get mixed up because you're hearing yourself slightly after the moment that you are speaking. It tends to make people think they are interrupting someone else, and can cause stumbles in speaking.
If you've done any studio recording, this shouldn't be too much of a problem. But if you've never done anything like that, it can be disconcerting. So if you're going to go down the path, be sure to practice recording yourself with this feature on. Get used to the way it works, and the output levels that you need for the recording vs what you are hearing.