Where the presence of hallucinations is a pretty clear indicator of a brain problem, this is not necessarily the case with regard to delusions. Delusions and hallucinations can and do frequently co-occur, but delusions can also occur just fine when no sign of hallucinations are evident. Brain damage or impairment is not necessary in order for delusions to occur. I'd like to say that you can use the bizarreness of delusions as an indicator of brain impairment, where the more bizarre the delusion is, the more likely the person exhibiting that delusion is to have a brain impairment, but this isn't necessarily the case. To clarify, it is the case that more bizarreness suggests a greater likelihood of brain impairment, except when those bizarre beliefs are widely held within the cultural group that the person happens to belong to. This is to say, if the delusion is a societal or sub-cultural delusion and not just an individual one, a person holding that delusion could be very much delusional but have a perfectly healthy brain and mind. From their own perspective and from the perspective of those around them sharing in the same culture, they would be sane. From the outside perspective of a different culture, they would appear delusional. Again, psychosis is in the eye of the beholder; it is a social process, and what makes sense within one society may not make sense in another.
Nowhere is this phenomena more apparent then when you compare cultures with high-contrast belief differences. A good example occurs in America today when people talk about evolution. On the one side of the evolution divide are scientists who take an empirical approach wherein the theory of evolution represents the best way currently known to account for the carefully accumulated and scrutinized data concerning human and animal origins. On the other side are people from faith-based backgrounds who practice bible literalism wherein if the bible says that creation happened as it says in Genesis, then that is exactly the way it happened. Each side's position looks fully delusional to the other side, I suspect. There is no way to say who is right without making a commitment yourself to one or another ways of understanding the nature of what is reality, so there is no ready resolution to the issue.
Psychosis is not the same thing as Psychopathy!
A few more things ought to be said before I close. You hear people talk about Psychopaths sometimes. Despite the similarity in the name, being a psychopath has little to do with being psychotic. Psychopaths are otherwise normal people (from a cognitive perspective) who for one reason or another have a terrible empathy deficit with regard to other people. They either cannot appreciate that other people have rights and feelings and lives that matter, or do not care for other reasons. Whatever the cause, they act as though they are the only important person in the world. Some psychopaths are skilled con-artists who are very good at leading people to trust them or fall in love with them, only to betray those people later on. Some become criminals. And, yes, some become violent murderers. So far as a diagnosis goes, psychopaths are more likely to be categorized as Antisocial Personality Disorders or Narcissistic Personality Disorders than as Schizophrenics, or Dissociatives. It's not hard to argue that psychopaths are delusional (due to their extreme empathy problems), but if this qualifies them for being psychotic it is a very different kind of psychotic than clinicians normally encounter.
Anyway, there is this myth that won't die that Schizophrenic and other psychotic patients are particularly violent, and this is just not the case by and large. Most of the psychotic patients I've know have been rather sweet people when you get to know them. Some act very strangely, and sometimes in alarming ways, but as a group they are not a particular threat. Its not that psychotic patients are never violent, but rather that they are not more violent than other groups of patients like substance abusers, for instance. I have this idea that people confuse psychotics with psychopaths and that may account for the persistence of this myth, but I'm not really sure.
Having made the points I wanted to make, I'll stop here. There is a lot more to say on the subject of psychosis (such as how it can be treated) but instead of making this essay overly long, I will refer people who are interested in additional material to our Schizophrenia topic center where a whole lot more on the subject is available.
Please feel free to ask further questions on the topic below at the bottom of the essay. I'll do my best to get them answered. And, should anyone who has experienced psychotic episodes read this, please add a comment describing your experiences so that they can become more clear and understandable to people who have not experienced what you have. When it comes to psychosis, having the right perspective counts for a lot.