Mental disorders can be of various type: Behavioral, depression, and recently heard that excessive laughing without any reason is also a kind of mental disorder. People usually hesitate to talk, considered as Taboo in India when someone says that I feel I'm in depression as no one suggests to talk about it and resolve.
One should always consult psychiatrist and openly discuss it with parents if they feel so.
Some symptoms might be:
1. Excessive Laughing
2. Not speaking at all
3. Being reserved at any public meeting
4. Sitting alone for hours and just staring at something unnecessarily
5. Multiple-thinking; a stage where person thinks atleast 3-5 things at the same time
The above mentioned symptoms are some which one needs to always look if he/she is feeling it for anyone or for themselves.