How/where can I see the current State Religion of another (computer) player? I cannot find it in the Religion View/Lens (whatever the button in the lower left corner is called) when clicking on their cities or in the diplomacy screen for that player.

It's in the Tenets screen, to the top right there's a list of all religions (existing and gone) by tier, if you mouse over one of them it will show who's its head, which empires follow it, number of followers and Holy Sites.