Let me start by doffing my cap, Sir. A GM who is willing to shrink oversized character ego's is ok in my book .
As to ways & means:
Your players are basically under investigation of taint and corruption with reference to dark pacts and whatnot.
I would start out by having them thoroughly vetted by the church. Have them recite the 'Hymn of Malleus Imperialus' or some such (e.g. test Imperial Creed) after having them totally shaven (yes, all over…) . Then immerse them in a large bath of holy water (test swimming). If you are really mean you can claim beforehand that if they survive they are guilty and if they drown they will be martyrs of the inquisition….
Oh, and have them lashed by a servitor causing a 1 wound reduction (e.g. the lashing weakens them) until they have proven themselves again in the eyes of the inquisition or sufficient time has lapsed for them to heal from such an ordeal.
Then the real tests start….
They are to be tested in some special inquisitorial vault which has many different rooms. In any particular sequence, let them confront the aspects of the chaos gods.
The players are each required to kill some fearsome beast and are offered a lasgun or a melee weapon and have to simultaneously protect some weak citizens. If they are particularly bloodthirsty (they pick the melee weapon and score max weapon damage during the fight) or ignore the citizens they are obviously influenced by Khorn…
The players are required to join some orgy whilst trying to extract information from a fellow reveler. They have to make willpower tests to withstand the charms of seductresses (or seducers) and or several tests against getting drunk or distracted…
Some kind of mindboggling test or magic/psyker test would seem appropriate. Perhaps they are offered secret knowledge to pass the test or access to a 'magic' weapon.
They are sent on some kind of mission (still in the vault though) and during that mission they are repeatedly exposed to illness and contagion (test toughness). Eventually, they will be infected and are given the choice of taking a semi-permanent toughness penalty (-3 would be fair) or accept the blessings of a suspect priest….
If the players come out of this with a mixed score, they are subjected to the final test: a meeting with a pet daemon. Bound by an Inquisitor, this pet daemon will interview the players with a series of riddles (easily found on the internet) to determine if they are indeed in league with a daemon….
Having gone through all of this, they would rather be killed by the next daemon they encounter than suffer through it again….