Intel / Counter-intel idea.

The idea is a simple structure that, once activated, will show you what intel a specific foe has gathered on you. I'll use SupCom:FA as an example.

Let's say I've started building a Paragon and suddenly a few scout planes fly near my base but are quickly shot down. Did they see my Paragon under construction? Obviously I want to keep it a secret as long as possible, but I have no way of knowing if it has been spotted. The idea here is that this intelligence structure would somehow let me see exactly what my opponent is and is not aware of.

I suppose a simple way of working this is that if you select the building, your view will shift to what your opponent can see of your base, then revert back to normal when you de-select it. I'm imagining something like the Facebook function that lets you see what your profile looks like to specific other people, except you're viewing your base through the eyes of the enemy. You could figure out if your opponent knows about your latest expansion or if he's spotted your incoming sneak attack.

To keep things balanced it should probably cost energy, either continuously or only when activated, and maybe have an effective range like radar does.

Not sure, I view intel as a recourse. You want as much as possible and preventing your enemies for getting any. If you infiltrate my base with cloaking units I would be able to easily guess my base have been compromised and deal with it accordingly.

I do not think it is a bad idea but I doubt it would fit into this kind of RTS game.

It should only show you the intel he gathered on behalf of where you know he have been.

lets continue your scout example.

an enemy scout flies over your base in range of your "whatDidHeSee" building, now if you select that building it will show what the units within it's radius are seeing.

That way, sneaky Intel gathering is still possible.

Why making it a structure instead of a UI tool? You know where the scout where, you know what its visual range is. The UI should be able to tell you if, yes, the scout did see your Paragon.
Which would, of course, only apply to the units you detected.

Agreed. Technically, you could figure it out if you were some kind of wizard who knew exactly how much units can see.

Sounds more like trying to use a modified spectator view, only that the "view" would be based on units that all players have visibility on. The effort to create that view accurately is in my mind more complicated than would be worth time to implement and debug.

It certainly sounds interesting, but not something that people would be using a lot.

Total Annihilation had the +showranges command that would show the ranges of your selected units when you held Shift. It would include all ranges, such as weapon, sight, etc.

You could just have a similar feature that would show the ranges of all units when you hold Shift, although it might run a little slow.

It would be more than just showing ranges. It should also tell you if a building has been seen or not.
Say that you are building a stealth base behind your enemy. Then, while you are busy elsewhere with a big fight, an enemy scout pass near it. You couldn't be there and check if it passed close enough to your buildings to see them, so an indicator on the buildings themselves would tell you if they were seen. It may also be useful for units like sneaky engineers or big nasty units as well.
Of course, this indicator wouldn't tell you about the stealth scout sitting in your stealth base and seeing everyone of your buildings.

Maybe something as simple as a "last seen by player A : 18s" tooltip when you hover or select the unit.

I think the way you figure this stuff out should be guesswork. Pay enough attention and you'll have a pretty good idea.