Signs. Why? A signal is a deliberate thing, e.g. you could signal someone to do something. A sign is passive. Someone showing signs of mental illness isn't doing it intentionally, therefore signs. When talking about illness "symptoms" is better.

"Signs" is better. Signals are intentional. This board is showing signs that it might break soon. Tom tried to pretend that his leg had completely healed from the injury, but his occasional winces and grunts were signs that he was still in pain. After the shipwreck, Bob built a fire as a signal to his rescuers. Sally kept giving him small presents as signals that she loved him.