Tainted Psykers are easily identified in battle, located in the epicentre of the warp rifts they maintain, and in fact during combat it is advisable to check if any given warp rift has a Tainted Psyker powering it, though not all do. When encountered, KILL THEM FIRST, as they can deal quite a bit of damage with their psychic attacks, are difficult to kill, and are generally keeping Warp portals open, bringing in a limitless supply of Chaos forces. It's best to take cover and eliminate them from range, as they sit at the centre of Warp portals and generally summon reinforcements which make physical approach difficult, but not impossible. A good way to deal with them is with a Lascannon from extreme distance, or with a carefully aimed and fully charged plasma gun shot. Their psychic attack is a multi-coloured arc of energy akin in appearance to lightning.