When you can start espionage, there is, in the bottom right hand corner of the screen, is an E in a box, if you press this it gives you some espionage options.
But I agree that the diplomat/Spy troop needs a comeback, In most times I've played the cost to perform any such mission is preposterous, stealing technologies upwards of hundreds if not thousands of gold/cash.
while a diplomat/spy troop could barely cost 30/40 shields and provided you could get to an enemy town you could steal a tech.
I do understand that this had its improbable drawbacks, which could be easily solved, One being that a race being behind in techs could steal a tech that was greatly advanced, I remember on a diety game civ2, stealing conscription when I could barely get chivalry.