Should be next to their name on the scoreboard.
From what I understand, it isn't a matter of having a certain number of Espionage points -- you have to have that many points MORE than the other guy has on you in order to qualify. So if you need 125 EP's to see technology research, and you have invested 125 in him, and the other guy has 5 EP's invested in you, you still need to get 5 more EP's until you can see what he's researching.
Please someone correct me if I'm wrong.
If you expend to many EP's against that civ you could lose that ability.
That's most certainly wrong, because "Can See Demographics" doesn't work like that - you only need x points, not x points more.
Have you continued to play? It could be Charlemagne just finished researching a tech.
They could have just finished researching their previous tech, why not?