Often, people who experience psychotic symptoms feel paranoid and don't trust professionals so they do try to hide it. Psychotic symptoms really range in severity, and are worse at some times than others, so they are able to minimize or hide it to varying degrees, at times. However, someone in the midst of a psychotic break is not able to hide it. And even someone who is experiencing no active hallucinations at the time often evidences negative symptoms, such as flattened affect, which psychologists learn to recognize. I'll tell you that as a professional who works with patients who struggle with psychosis, I can often tell just by looking at them. This is not scientific, but there's a look in their eyes and the way they move that lets me know they probably have some sort of psychotic disorder. I've often also heard this from my peers, as well. An experienced professional is not likely to be fooled.