When a person has a mental disorder, it cannot be 100% determined without the intervention of a specialist. But there are obvious signs: incoherent speech (the sentence is constructed correctly, but the words were chosen incorrectly), poor sleep, hallucinations (auditory and visual), panic attacks, the desire to kill or hurt, love for Russian turnips, are in financial pyramids. The last 2 signs are the most obvious.
Dear question author. The problem is that there is no such general concept as "crazy." But in everyday life, of course, it is, and it means that a person has begun to develop or has already developed one of a number of mental illnesses. And there are quite a lot of these diseases, and everyone has their own symptoms. Not every mental illness is accompanied by delusions and inappropriate behavior. But in almost all cases, the behavior of a person who has developed such a disease seems strange and illogical to us from the outside.
The period when an outwardly healthy person begins to manifest a mental illness is called "debut". The first thing that can alert a person, if this takes place - his behavior changes, you stop recognizing him. He, suddenly, becomes careless, starts up his home and his appearance, for no apparent reason, cuts off external ties, leaves work. Or, on the contrary, he becomes exaggeratedly pedantic, nervous about any insignificant disorder. Or he begins to carry nonsense, devoid of any meaning for an outside listener, but clearly filled with the deepest meaning for himself. He may experience mood swings, he may, for no apparent reason, either come into extraordinary emotional excitement, from exaltation to overt aggression, or fall into a state of apathy and internal devastation.
Since this is just a small list of what we are. we can observe from the outside and what can give us a doubt about the mental adequacy of a person, then we should not make hasty conclusions. If you are now talking about a specific person, and not just abstractly interested, then it would be nice to try to find out from the person himself, who knows - maybe his changed behavior has explainable grounds? Maybe has something happened in his life that explains the changes in his behavior? Who can say for sure, maybe a person has lost a beloved relative? Having a hard time breaking up? Got a problem with alcohol? Are you in serious financial difficulties? Depressed? Addicted to drugs? Got into a sect? There can be a lot of things here, and the external manifestations are often very similar, and the question does not contain enough information.
In any case, if we are talking about a real person, but at the same time you do not have close contact with him, do not are a person close to him, whom he trusts, then the chances of real help will be small, because no one has the right to forcibly impose psychiatric help as long as a person is sane, and only a court can recognize him as insane (and this, too, believe me, does not promise him anything good). And to convince him that he really needs such help, he needs the person to trust you, then maybe something will work out.