Knowing your opponent early on through renaming your capital

With the new patch of CIV 4, you can no long know your opponent at the start of the game through looking at F9 city screen.

I just found another way. . Play the game for one turn. Then try to rename your capital to the name of other civilization's capital. For example, if you can rename your capital to Athens, then you know Alex is not in your game since the name of the city can not be duplicate. If you can not, then Alex is in. Of course for the civilization with multiple leaders, you can not know which leader you are dealing with.

This is probably most useful at the early game to decide that whether you want to pursue early religion. If at higher level, your game is packed with Saladin, Gandi, Monty, Issy etc, then you can forget to get an early religion.

Why not just open world builder and look or choose your opponents yourself?

If multi player couldnt the other civs just use their own custom names for their cities, therefore this wouldnt work?

They could, but in MP is the place that this would give an unfair advantage to the unscrupulous. Of course, in most MP envirnonments people pick non-random civs beforehand and everyone knows who is playing.

So it's not a big deal.

This is quite cheap... But it's also quite cheap that if you want to found Buddhism, you have a roughly 0% chance of doing so if Isabella is in an emperor level or higher game with you. I won't do this myself, but since I consider the religion founding game outright unfair to the player on higher difficulty levels, I won't chew anyone out for it.