Originally Posted by
factotum
Demographics and your current score (there's an option to display the score table at the top right of the display) are your best indicators as to how well you're doing compared to the other civs. You can also look at the score in the Diplomacy screen, and that has the advantage that it'll break the score down into its constituents if you hover over it--so, if your enemy has a tech score of 140 while yours is 120, you know they've got a good few techs more than you have!