People having some psychological or psychiatric problems, very often do not realize that, or they may as well consider certain symptoms to be a result of some other problems, that is why a consultation from a doctor may be necessary to not mistake.
Slow (compared to usual) thinking, speaking, and body movements
Experiencing problems with decision making, focusing on current tasks, or remembering things
Unproportionate anger outbursts occurring over even the least important matters
Problems with sleep, such as insomnia or, on the contrary, excessive sleepiness all day long
Fatigue; even the smallest activity causes excessive tiredness, and working requires extra efforts.
Loss of interest to activities that were once favorite
Decreased libido, loss of interest to sex and romance
Sudden physical problems that have no explanation (for example, constant headaches while being absolutely healthy)
Constant anxiety, worrying over everything, inability to interact with people normally
Constant sadness, guilt, or fear; blaming yourself for mistakes made in the past; episodes of self-reproaching
Either reduced appetite, or increased craving for food (up to bulimia)
Suicidal thoughts/attempts, frequent thoughts about death, committing suicide (well, the latter is not rather a symptom already)