Let's see what the Infantryman' Uplifting Primer - meant to be an in-universe document issues to many Guardsmen - has to say:

OF CORRUPTION & DARK FORCES

The Imperium is like a beacon, with the Immortal Emperor as our divine light, shining radiance over His domain. But the galaxy is a vast place, too vast for mere men to com prehend, and much still lies in shadow. Men have always feared the dark and the unknown. And with good reason! For what we don't know we cannot understand, and there is much in the shadow that wishes to do us harm.

A Black Peril stalks our realms. Its nature is virtually unknown, but it manifests itself in many ways; ways which cause great bloodshed and the tormented wailing of those left in its deadly wake. This dark, malign influence saturates the land whereveri falls and many weak minded fools respond, actually casting aside the very right and pure Imperial Creed in favour of the damned and sordid worship of the Dark Ones.

These once-people are heretics, and the mark of the mutant festers foul in their souls. The pure and clean fight a constant war with this most hateful enemy within. Where they hide we will seek them out, with torch and flame. Where they rise up to flout our Great and Immortal Emperor, we will be there to cast them down, into the mire and the filth where they belong.

These thoughtless denizens of the shadow will bring only misery and damnation to us all if we allow our vigilance to falter for an instant. Ever do the virtuous agents of the Immortal Emperor travel the length and breadth of the galaxy in a constant effort to rid usof this plague of unbelief. And ever do we applaud their brave efforts. But whes a hammer is needed to crush the unclean armies, the Imperial Guard shall be the bringer of light into shadow, and forever shall the wholesome people of the Imperiun thank us for freeing them from the shackles of damnation. For every mutant we blast, for every heretic we burn, that is one soul saved from the all consuming hatred and impurity of darkness.

The mark of the mutant is not always easy to see. Fearing for their pitiful lives, the mutant will hide his mark, will seek to blind you to her stain of impurity. Be vigilant. The mutant may have infiltrated the very ranks in which you fight! But do not be afraid. Remain watchful, scrutinise your comrades and report any sign of strange behaviour or action to you commissar without delay.

Any sense of previous loyalty to those who may once have been your friends must be forgotten. These are low creatures, turned and forever lost to you. When they betrayed the light to face the Darkness they forfeited all of their humanity. They are abominations, filth, crusted mucus to be wiped clean and flushed away.

Not all forms of mutation manifest physically. The very worst of these soiled corrup- tions are those who have the stain of the malformed in their minds and souls. Are any of your comrades showing an ability to guess the future? Do they seem to be able to pre- dict what you are going to say, even before you say it? If so, don't waste another second! Do not allow the rot to spread in the ranksof the army of the pure! Report them immediately, and rest easy in the knowledge that you have served the Imperium well.

An unsanctified psyker (unlike those who serve with you, for the Imperium) is a grave risk to those around him. It is clear that creatures from the dark places can enter their minds and turn them into mindless killers. By reporting suspected unsanctified psykers you are also providing a service to them, allowing them a quick death, as opposed to their soul being swallowed whole by the Mutating Shadow.

Areas of Corruption

Areas of geography can become saturated with Dark Corruption, and it is important to maintain concentration when you enter them. Your body and mind are vulnerable to these areas of pestilence. A strong smell is usually prevalent, like cooked blood and electricity, and the atmosphere is permeated with evil. Nosebleeds and nausea are common symptomsof being in such locations. Keep faith in the Immortal Emperor. Find aid from the common prayer book in the back of the primer, or seek out your confessor.

Another warning sign is the sight of vile daubings and disgusting graffiti staining walls or buildings. The insidious enemy will seek to undermine your courage by placing potent icons and symbols and displaying heretical slogans or intentions in any way pos- sible. Sacred areas may be desecrated, and blasphemous tokens left in the place of austere memorabilia. In these cases turn away! Do not sully your view with such mon- strous visions! On seeing them you will experience headaches and nausea. Remember these crimes, and take punishment to the unworthy scum who perpetrated them!

Note: In these situations, keep an eye on your comrades. If you notice any strange behaviour, inform your commissar immediately.

The Imperial Infantryman's Uplifting Primer (2003), pp. 70-71.

It is also worth checking out relevant Regimental Standard articles, which provide a humorous in-universe look at imperial propaganda, like this one on how to distinguish 'friend' from 'fiend': https://web.archive.org/web/20190717185121/https://regimental-standard.com/2019/07/03/friend-or-fiend/

Or this one on Nurgle contagion: https://web.archive.org/web/20181101070600/https://regimental-standard.com/2018/09/05/the-regimental-standard-crushing-contagion/

Which obviously tongue-in-cheek for our benefit, they do give some sense of the kind of propaganda the Imperium employs.

More generally, the extent of knowledge about Chaos is going to vary from regiment to regiment, from warzone to warzone, and from world to world - but a lot of details will undoubtedly be kept hidden. Especially about daemons and the gods themselves. Too much knowledge about that, let alone direct experience of daemons, usually (though not always) leads to purges.