A large number of mutations are considered psionic mutations. These mutations are ones that affect or sense the psionic centers within the human brain. In short: they read or alter parts of a human's mind.
Some mutations that fall under this category are:
Since psionic mutations are based on projection and reception to the psionic center of the human brain, those with psionics mutations often have varying levels of awareness of the minds surrounding them. Note that this does not extend to non-sapient minds, which generally do not have a psionic center.
The psionic center is a portion of a human's brain closely tied to the limbic system. It allows for projection and reception of psionic abilities. This portion of the brain can be damaged through injury.
It exists in humans. Studies still need to be done on other higher primates. Lower primates and other mammals do not have a psionic center. Some mutants are able to affect animals, but this is not considered telepathy in the same manner as person to person communication, although it can be very similar.
Many of the actions of the psionic center are fueled by glucose. It's not uncommon for a psionic pushing their limits to go hypoglycemic. Experienced telepaths often carry juice or other forms of readily available glucose to combat this.
Psionic manipulation leaves 'fingerprints' - or signs of the manipulation - which another experienced psionic may be able to pick up on. Fingerprints are freshest just after the manipulation, and fade over time. Psionics can only sense the fingerprints affecting the area their mutation also affects (ie memory, emotions, dreams, etc).
Fingerprints can indicate what sort of manipulation occurred and, with enough familiarity, who was behind it. Generally a psionic's ability to read fingerprints depends on their skill, on the skill of the person who left them (a particularly capable telepath, for example, may be able to leave minimal fingerprints with serious effort), and on their familiarity with the people in question.
One important notes: fingerprints are associated with projective abilities, as projection implies the manipulation of someone's psionic center, while receptive abilities -- typically telepathy, empathy -- are limited to sensing what is occurring in someone's brain. If the brain is a glass, receptive mutations simply view the glass, while projective mutations pick up and move the glass, leaving behind a fingerprint.
Psionics can default to receptive or projective or neither. It is much more common for a psionic to default to receptive or neither.
Reception is a passive ability: hearing, rather than speaking. With training, one can learn to hear better. Thoughts can be chaotic, and a skilled receptive telepath will be better at isolating a single mind or a single line of thought.
Projection is active, and as such requires more of an effort from the psionic. A young psionic who doesn't know what they are doing is at risk for burning themselves out. While all psionic activity burns glucose, projection burns it much more quickly.
Most psionics are both receptive and projective, although they will often tend to favor one or the other.
Psionics are often limited in range. The more complicated mutations, such as illusions, are often limited in the range of dozens of meters, while few telepaths can range over a hundred meters. They may also be limited by line of sight. They are often also limited in terms of how many minds they can effect at once. Their mutations require focus that can be broken, and fuel in the form of glucose. Low blood sugar can lead to headaches or worse.
A default receptive telepath would be constantly hearing others' thoughts unless specifically expending the effort not to listen. A default projective telepath would be continually broadcasting without a similar effort.
Telepathic conversation between a telepath and non-telepath depends entirely on the telepath's mutation, which means that conversing depends on the telepath's intention. IE, there is no way for a non-telepath to 'shout' to a telepath unless a line of communication is already open.
In RP, telepathic conversation is displayed between two less than and greater than symbols, << Like this. >>
It is not possible for a non-psionic to truly shield against a psionic, but it may be possible to distract, disturb, or otherwise put off a psionic who is not determined.
Learning to focus one's surface thoughts on something nonsensical, like the sizzle of a frying pan or the roar of running water, can keep receptive telepaths at bay. Likewise, focusing one's thoughts abruptly on something distasteful (such as an image of extreme violence or other disturbing thoughts) can sometimes jar a telepath out of contact. Neither of these will bear up if the telepath is determined.
Another psionic may be powerful enough to protect their mind from psionic interference, but generally only in their area of specialty. That is, an empath could shield against a telepath's efforts to manipulate their emotions, but they could not shield against manipulation of their thoughts or memories.
The Astral Plane is not a physical location. It is the collective unconscious of all of humanity. Cerebro taps into this, and it is where dreams take place, if they can be said to have a location. It has been used as evidence that psionic potential exists in all humans, mutant or not. Because it is not a physical location, it has no real 'appearance' of its own but shapes itself according to the will of the viewer(s) entering it.
While everyone can manipulate the Astral Plane to some degree, with non-psionics able to choose their own 'aspect', projective psionics are very much at home here and able to manipulate it following similar but enhanced guidelines to their mutation: empaths can manipulate emotion, illusionists can manipulate visuals. It is much easier to manipulate the astral plane than reality. The unconscious mind is more suggestible and has fewer barriers.
A non-psionic on the Astral Plane may not be aware of what is happening at first. The experience can be likened to lucid dreaming as the non-psionic becomes aware.