So I want to have all 60 Oblivion Gates closed and am currently in the middle of the MQ quest where Martin asks me to get him a daedric artifact. I believe the number of OG open at this moment is 50 and is therefore the maximum it can ever get. Now I have 54 OG closed (including all the city gates plus Kvatch gate) so that would leave me 4 gates short of the goal (60 total Oblivion Gates - the Great Gate - the Bruma Gate = 58). When I look in the map I seem to think I've closed them all but how do I know how many OG have I discovered and how many are displayed in my map? Maybe there is some console command to point me to an open gate or a mod or something? I really don't feel like making an excursion around Cyrodiil and I don't have the paper map too.

I don't think they open until you're close to them, so there would be no way to locate a remote open gate. IIRC each gate runs the same script when you're in its cell that essentially says "Has the limit been reached? If not, random chance that I open".

So that means that all you need to do is go to any part of the map you haven't explored since they started opening and you should find one pretty soon. Of course, it's then a question of remembering where you need to revisit!

If you're a PC user I believe the Map Marker Overhaul mod will show you different markers for open and closed gates, even if you install it after closing them. That may help you relocate the ones that opened, but you didn't close them while you were near.

This. At that point in the MQ, there's a 50% chance that a gate will open when you enter its cell, so just keep wandering the countryside until the last few open up.

There are 90 set locations for random gates, and not every cell will have one, so you might be walking for a while and have to backtrack a lot. I remember when I tried this, I spent a lot of time in the eastern marshes. Only 10 appear during every playthrough, the other 50 are just chance. If you're not having any luck in one area, go to the other side of the map and try there for a while. There's even a few up in the mountains.