Learn English – How to confirm a Skype meeting

The most authentic-sounding is the second.

Which room are we meeting in?

Option 1 is not valid. You are meeting (I imagine) in the room, and not "at" (i.e. outside it).

Option 3 is valid but likely overly formal.

Option 4 is not grammatical. "Where" and "in" do not play nicely together. When combined like this, they become "in which". So, you could potentially say (though I maintain it's formal/archaic): "Where/Which is the room in which we are meeting?".

In ascending level of informality, you could ask your colleague to give it a look, give it the once-over, eyeball it.

So far as I'm aware, there's nothing particularly informal about look it over, but you might prefer to be more explicit about exactly why you want him to look at it: ...so you can check it before release.

Purely my own opinion, but I think check it out is much more informal (whereas check it over doesn't seem unreasonable in a formal context, but they have slightly different nuances anyway).