Treatment for mental illnesses, including that for schizophrenia, is an imperfect deal. Those who are in treatment will almost invariably have some residual symptoms. A person with schizophrenia will sometimes be delusional while taking medication. Sometimes the dosage is wrong, either too low or too high. (Too much medication can be just as bad, sometimes worse, compared to too little medication.) Also, antipsychotic medications do not always work. About a third of people with schizophrenia aren't helped by medication.
When someone has a lot of experience dealing with their illness they can sometimes self-assess. Yet this is not always reliable.
A person with a psychotic tendency would do well to have several layers of carefulness-like a failsafe system, to prevent getting excessively delusional. For example, self assessment could be a first layer, and this could be followed with monitoring by one's significant other (or perhaps a roommate.) The idea is to have people in your life who know you and who you can share thoughts with, so that you can get another opinion on possible delusions.
Keeping secrets from one's psychiatrist prevents being helped. The point is to catch one's delusions before they proliferate. If psychosis while medicated becomes too severe, it increases the likelihood of noncompliance.
The question arises: how do you know it when you're delusional, and what do you do about it?
Moreover, when are people really "out to get me," and when is this thought merely a product of the illness? When is a person dealing with the "real extraterrestrials" versus the ones that the person is imagining? When is someone really a famous person, and when are they having "delusions of grandeur"?
I am helped by the "I don't know" axiom that I have adopted. This is the admission that my knowledge is limited. If I adopt an "I don't know" stance, I am less likely to act on a belief that may be inaccurate. Prevention of acting on delusions is like declawing a tiger. You still have a tiger but it is no longer mauling people. Thus, if a person with a delusional disorder learns to regulate their actions, it prevents their delusions from doing external harm. (In order for this to work, it is necessary to have some idea that a thought is questionable.)
A "delusional system" is a set of interwoven delusions that take the place of the perception of reality. A person with psychosis may be emotionally attached to their delusional system. The delusional system may have elements that reassure a person, that make a person special, or that shield a person from difficult truths. Delusions tend to be beliefs that are very unusual and that do not ordinarily seem credible.
Getting over a delusional system usually requires medical treatment, e.g., medication, but can also require that a person "give up" a belief to which they may be emotionally attached.
Because I have experience with techniques of mindfulness, I have learned not to be emotionally attached to delusions. This is an area in which meditative practices can augment drug therapy. When someone eliminates their emotional attachment to delusions, and is not afraid to tell a psychiatrist or family member what the delusions are, they have a better chance of becoming nearly delusion-free.
Delusions sometimes have power because they hijack the pain and pleasure capacities of the brain. Delusions multiply when kept secret. When a person with schizophrenia takes the step of relinquishing the emotional attachment to delusions, they have eliminated something that reinforces their illness.
I will often use my wife or my psychotherapist as an external "reality checker" where I ask how realistic a particular thought sounds to them. I don't share delusions with strangers or with members of the general public because they would not understand.
Schizophrenia is insidious partly because it warps all of the perceptions. Thus the psychotic person will believe that they have a lot of evidence to support beliefs which they do not realize are false.
If I were taken off medication, it would not matter that ordinarily I know the difference between a delusion and a truth. I would become delusional from being taken off medication because the basic reality-finding ability of my brain would stop working. I would forget all of these wonderful thoughts and techniques that today help me think pretty clearly. If I lacked medication, I would become more and more delusional until I reached a point where I would need external intervention.
Getting the brain's "hardware" to work by administering medication is the first step, and only after that can a person work on the errors in "software," originally produced by a malfunctioning brain.
There are some strategies for alleviating delusions in addition to standard treatment that might surprise you. Getting distracted by something calming and enjoyable can reset the thoughts, which can allow reality to filter into the mind. I once got over my delusions when the movie "Field of Dreams" was played in a psychiatric ward. I was released within a few days of that.