As an employer, manager, or supervisor, it is not your job or your responsibility to diagnose a mental health problem. However, being aware of the signs that suggest someone might be experiencing a mental illness is important. Mental illness includes a broad range of symptoms and behaviors, and it is not easy to determine whether someone is mentally ill. One key indicator is that someone may begin to act uncharacteristically; an energetic person may seem lethargic for a considerable time, or a person who is usually mild may make grandiose claims about their abilities.
Behavior changes such as these may reflect personal difficulties that will be resolved quickly. They may be signs that the person is no longer happy in their job. The individual might be going through a particularly stressful time in their life for any number of reasons. These behavior changes might, however, indicate that the person is experiencing a mental health problem that goes beyond being "stressed-out" and that requires professional help.
If an employee’s behavior is a workplace problem, talking to them privately may help you determine whether mental health is a factor. You may be able to encourage the individual to get help and/or request appropriate accommodation while they deal with their mental health issue.