Warhammer 40K Fluff Discussion XVI: Where the Ordnance is Hand-Delivered!

[Cheesegear]

How exactly does your average planetary governor know how to tell psykers from non-psykers in a planet's population?

Until the Psyker starts doing things, there is no difference.

And how do the Black Ships know if a world is doing its due diligence to get them?

They don't.
But it's in the Governor's best interest to be as diligent as possible. Rogue Psykers can doom your planet.

Would they use a percentage estimate?

They use a Tithe system.
"By the time we come back (every 50-100 years), you should have [x] Psykers, execute the rest."

I initially thought that they would wait for an obvious manifestation and then swoop in...

That's exactly what happens. The alternative is swoop in before they manifest...Which is like...Minority Report shenanigans.

Also, what's to stop a planet government to just outright kill anyone suspected of psykery? Would they get punished?

Depends how backwards the planet is.
If you accuse someone of being a witch, that might just be all it takes. Something, something, real world.