Signs? Sometimes there are none. Sometimes changes in personality are complicated by attempts to self-medicate with substances like alcohol or drugs or excessive behaviors such as spending money wildly or doing something uncharacteristic such as making sudden major decisions about lifestyle or beliefs. These are just a few examples. Sometimes you may have only a vague feeling a problem exists; not everyone displays telltale behavior changes. To complicate matters, not all changes like this are serious, especially in the volatile teen years, though young people and children can become mentally ill.