To see negative enchantments, I believe you have to go into the city details screen and mouse over each of the statistics to see what is affecting each of them. Since there aren't that many particularly bad negative enchantments in the base game, I generally don't bother because this is too much effort for determining if a city has been cursed.

Not as far as I know, but I may have missed a dispel somewhere or other. Only three ways I know of to check would be to wait for someone who knows about a dispel to post here, read through the in-game Hiergamenon spell list to see if a dispel is in there, or check through the game files to see if you can find a dispel.

Maybe this is something that the new developers should fix.  Why Not?

Some sort of pop-up saying, "hey, your conclave has been cursed" wouldn't hurt either.  I do like the idea of something visual on the city bar as well.

I believe the number of enchantments on the city include positive buffs, negative buffs, enchantments on units stationed there, and something else.

(yes, I have been frustrated by this, also.)

One clue, though, is when you see something that does not make sense in your city list (like unrest which is higher than anything else, or cities which are not producing when they should have something in their production queue.)