Quote Originally Posted by factotum View Post
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!
If you can get trade routes with them, that can give you indications as well. If they give you a lot more beakers/turn than you give them, they have outteched you. If you get relative gold parity from the routes, the cities(and presumably the whole civ) are similarly developed to your own.

From there, if you've gathered that you are ahead, if you go to the trade/diplomacy screen and notice that they have low gpt on top, they may be ripe for some plucking.