Witnesses are straightforward: blackmail multiple members, or get some plants who are "new to the ward". Might take awhile to build up a ward-wide infiltration network, but it's likely worth it.
Hell, we're talking about the same demographic that routinely falls for MLM schemes like dōTerra. Once you get a bit of a nucleus, a ward's espionage machine can very well be self-maintaining and decentralized, using peer pressure and some judicious application of scripture to guilt-trip people into being informants.
As for evidence: that's the whole discussion here :)
> There is also (to us) scripture that says Church discipline can only deprive one of membership
Which is all that's really needed to be effective leverage.
Edit: Which doesn't make it easy or less real -- but knowing it really helps me.
pps: I have lived in and visited a variety of places, and I read a lot. The Church is the best influence in my life, helping me and others I know, in some very hard times. A lot there.