Well, there are hard facts that show that it is working, either the tresury and spending drops in the case that they counter with propaganda, or you see star systems defect more and the pop growth slows down. I've gotta start doing this in some of my games where I've got a race that looks to be a bit more trouble to wipe out. Anything else like that which is mentioned earlier would just be eyecandy... though there's nothing wrong with that .

Even if you see things like morale or funds dropping, a lot of things can cause that, there's no way to directly tell how much of it is by you.

I know the stuff I mentioned would be just for show, but it would still be nice to see the game in some way acknowledge the money you're spending.

Perhaps you should gain reports on planetery improvements destroyed by your pet terrorists and rioters in the destabalised civ.

How dare you, terrorists indeed! they're freedom fighters, and don't you forget it or they may come and visit you early one morning for a spot of 're-education';)

My strategy is to start funding espionage and destabilization early in the game, as soon as all the players have been contracted in some way. I push the slider on destabilization until the first number comes up - usually around 1 or 2 bc. I push the espionage slider to be twice the destablization slider number. I then leave them there throughout the game. By the time the game is almost over, I am stealing techs almost every other turn and the amount being spend on the one or two AIs left in the game is around 30-45 bc per turn and half that for destabliation.

BTW, I don't usually have to steal the Capital ships, they are usually given to me by the AI for free. YOu get them for free when you are superior in diplomacy, economy, and military. But, I almost never fight a battle except in self defense. My influence just takes over the galaxy and they flip. I do like to terror star their last planet though.
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I use the same tactics as Weyrleader. Never waste your ressources on wars - conquer by ideas (influence). I don't have to pay maintenance for a horde of ships - I build developed infrastructure instead. In a moment, if something happens (like Space Shark, pirates or alike), I can build a fleet, which will be strong enough to solve my small problem.

You can be both a freedom fighter and a terrorist. Freedom fighter is what you are and terrorist is your occupation.
ie terrorism is your method and freedom your goal.