The symptoms of a person who is suffering a nervous breakdown are quite noticeable, and we usually describe them by saying he or she 'lost it'.
The symptoms include erratic behavior which seems to lack any continuity or meaning to an outsider, spontaneous outbursts of emotion, inability of social interaction or even the inability to form coherent thoughts and sentences. How long this state lasts and the severity of it fluctuates depending how well the person picks up the pieces and what kind of new identity he or she chooses to create.
The symptoms of a person who is on the verge of having a nervous breakdown are also quite noticeable and characteristic. Such people are almost obsessively focused on the things or work that stresses them, and they are unable to deal with even the smallest distractions. They are trying to hold onto their sense of control very tightly, but they feel it slipping and this terrifies them. They might lash out in anger or fall into depression at any time.
The best way to help a person who is suffering a nervous breakdown (or is about to), it to make the break as clear, painless and quick as possible, and to provide a loving and supportive environment for mental and emotional self-regeneration.