here's an idea:
a) whenever your activist is out on the streets (causing trouble, selling brownies and so on) they can be taken. if you pay close attention you can notice this, otherwise you might be out of luck. Once taken the liberal undergoes an interrogation by the CSS, in much the same fashion that you interrogate people, except you won't see this. finally when the liberal is turned, he will be returned to you. from that point on, the CSS-sleeper will only bring in 30% of the cash she is supposed to, their liberalizing work is ineffectual (at all), and any mission she is part of will draw CCS attention (they will be the conservative response, and they will do it quicker).
b)so... if you suspect someone (anyone) is a sleeper, you can have your guys bring him down to interrogation. 2 thing can happen now. Either the guy is guilty, in which case it's a regular interrogation, allowing you to bring down their wisdom as a bonus, and giving you your member back. Or the guy is not guilty, in which case he will never convert, and you will have to execute a liberal member, or risk having him run away to the police (like any kidnapped person would). CCS can do the same, and root out your agents.
c) just though of it: once your sleeper (the one you placed in CSS) is deep enough, he can attempt a risky business of investigating where the intel comes from. if successful, he will reveal the name of the sleeper CSS has in your midst. the alternative is that he gets caught and it's bye-bye sleeper.