Is there a way to find genestealer in disguise?

I got an auspex (thinking it can help find the dif in bio between human and nid, dm says he doesnt think it can be used that way... maybe there is a way to tune the machine spirit in a paticuliar way?)

we got an apoticary and a librarian too if it is of any help...I'm an assault marine

The apothecary should be able to do a Medicae screening to detect the alterations to DNA.

Librarians with Telepathic powers might be able to detect the inhuman thoughts or domination if they're scanning a hybrid's mind.

Hybrids (except for late-generation) clearly look somewhat inhuman. Disguise vs Awareness or whatever the opposed test is.

I would not give the auspex any applicability in this situation.

How the apo scan work?

Test an individual with a Medicae roll, GM determines the difficulty. The IG can help with this if there are any IG Medicae personnel present as all IG undergo a gene scan, so their surgeons probably know what to look for too. Rest of the KT herds people up for testing. Anyone who breaks from the line and runs gets a bolt to the back.

The mere threat/rumor of this caused a group of hybrids to attack in my gane. They knew they couldn't escape or avoid it so they went after the Astartes and civilians present.

TEP covers the kiss, and basically was a more-than-challenging medicae test with the right gear (using a diagnosticator or the like, requires medicae), or using the Reading power. It also suggests the GM make the rolls for the player, as they shouldn't know if they've succeeded or not.

I'd expect full on infestation to be something similar, probably getting easier the closer to being a purstrain the individual is as the more alien they become.