They Came from Beyond
Mind Control Immunity
In response to the question, "How do the people that are immune to the Mind Control fields (i.e. the PCs) actually know that they are immune to the effect? There has to be some way of figuring this out, without the “shove them in and see what happens” test."
To which, tummult responded, “People who are immune know they are for two main reasons. They were in a city when the crystals fell or they went into the city to retrieve something or someone very important to them. The actual number of Immune persons is much higher than what is known, but like you said, they aren’t eager to experiment.”