It's not called "Correct answers" thread, now is it? It's the quick answers thread ;)

How do you win a conquest game on anything standard size or above? I've been having a go but everytime by the time I knock a couple of civs over the unhappiness from razing so many cities is crippling. Should i be aiming at some policies or certain civs? None seem to offer many benefits.

You only need to capture the capitals. Basically you just focus on winning with culture or science, and if the AI is down to two capitals you simply capture them. It's not that hard.

I've captured capitals though and the civs are still knocking about. If they re-take them does it return to status quo or are they counted as dead in terms of domination?

I don't know. But you can check that in the victory screen.

Quick question:

Assumptions: (Correct me if I am wrong)
I can see the total unhappiness from all of my cities via hovering over the happiness face.

Happiness buildings only have an effect if there is unhappiness from that city left, that is not covered by a building yet.

Is there a fast way to see whether an additional happiness building will have an effect if I build it?

Durrr...

I don't know. I think I check how happiness for a city is calculated in the city screen.

And that is where I didn't find it yet...

Not from what I can see but perhaps there's a mod for it? similar to the mods that used to offer for Civ 4 Gotm?