A person who is delusional believes what they want to believe even in spite of obvious evidence that proves their beliefs are false. No amount of reasoning or logic would prove them wrong. The beliefs may not be consistent with their usual educated upbringing. 

People suffering from delusional disorders may believe that the actions of others are intended to cause them harm and they are being singled out. Except for their own "iron clad" beliefs that defy reality, people with delusional disorders can appear normal. The meantal impairmeants that affect people with schizophrenia don’t usually show through those who have delusional disorders. 

Regular functions such as motor skills, keeping appearances, and behavior in general are usually not impaired, except for their own reasoning associated with the delusional disorder they may function rather well. However, problems tend to increase because of the delusions. 

At many times it is easy to spot the symptoms of a person with a delusional disorder:

-The type that believe that people of importance (famous people) love and admire them is the Eroctomanic type. Usually they constantly send letters and gifts or harass the target of their delusion. 
-The suspicious type is called Persecutory. People that suffer from this type of delusional disorder are convinced that there is a conspiracy against them, they could believe that there is always somebody watching them. 
-Grandiose type possess great abilities, power, and believe they have special knowledge, wealth, and riches. 
-Somatic type: their delusions focus on physical defects, disease, or conditions of the body. A person with this delusional disorder may see parasites and believe they smell horrible odors. 
-Jealous types in their delusions believe their spouse is having an affair, without basis. They may search for "evidence" to support this delusion. 
 

Delusional disorders usually start during mid-life but symptoms may last for years. Most people attribute delusional disorders to psychological and personality issues.