The game makes it hard to diagnose mana toxicity in your servants without Mind Reading, nor is there any apparent way to help things get better. As such, I think methods for dealing with toxicity would be good.
Method 1: Mana Extraction. Making out with servants draws out mana already - so it would make sense if some of that mana was of the toxic variety. Toxic mana doesn't contribute to mana reserves, being useless. The post-coitus report should note the amount of toxins removed.
Method 2: Extraction Potion. The extraction potion forcibly removes mana from a creature - at the expense of their energy, and possibly causing stress if energy levels are too low. Consumes 60 energy.
Method 3: Mana Farm. The mana extractors on the farm isn't just for getting mana, it can remove toxic varieties from the folks who are on the farm. While not fast, the farm extractors don't cause extra stress by themselves. Removes about 20 toxins each day.
Method 4: Guild Assistance. You can pay the mage's guild to remove the toxins. They keep any good mana for themselves, and scale their pricing according to the quality of the servant and the degree of toxicity.
Giving toxified servants an unhealthy aura would be useful. Unlike with mind reading, there are no specific numbers attached to the toxicity, just a general impression of how bad things are. Their portraits could have a "glitter" effect, color corresponding to nastiness.