Descriptions of settings will change the tone. A young child plays in a shallow stream as his mother watches from nearby. The tone is nostalgic, comforting and happy. The stream starts to rise quickly, turning into a flash flood. The child clings to a rock in the middle of a raging torrent. The tone has shifted to one of horror.
Characters' actions can change the tone of a piece. If a romantically involved couple are having dinner, the tone is romantic. If another man enters the restaurant, the mood can quickly shift from romantic to suspenseful -- especially if readers know the second man is having an affair with the female character.
Characters' actions can change the tone of a piece. If the woman suddenly leaves her date and passionately kisses the man while her date looks on, the mood becomes vengeful.
Dialogue can also change the tone of a work. Perhaps the date calmly states, "I see you've met my brother." The mood has now changed to shock and surprise.
A character's attitude can change how his words and actions affect the tone of the story. What if the date says, "I see you've met my brother," in an irritated manner. Instead of the tone being one of shock and surprise, it would be one of contempt.
Irony can affect tone. If a character says, "I love you, too," it would normally set a romantic tone. If those same words were said by a character who has just been betrayed by their romantic partner, the words could be a contradiction to what they really mean. This use of subtext to create irony affects the tone, which in this case would be angry or regretful.
A Single Word
Any part of speech can be used to change the tone within a piece of literature. When looking for tone shifts, look for key words. Consider how the nouns "man" and "freak" create entirely different feelings in the reader. The verbs "rain" and "downpour" also convey different tones. Does a character go about his work "carefully" or "doggedly?" All it takes is a single word choice to produce a tone shift.