What types of clues in a poem let you know that you should use a different tone?

The use of words, theme, mood of the poet and uses of various phrase can tell that you should change the tone .

Explanation:

Some words that describes the mood or phrases of a poem might be showing through his poetry about romantic,optimistic, realistic, pessimistic, mournful, gloomy, sorrowful, etc. Some words which describes about the tone of a poem might be: humorous, amused, serious angry,playful, cheerful, sad, happy gloomy, etc. the tone of a poem, gives a hint that you must feel about the writer's attitude toward the subject  of the poem or or the audience.The Tone generally refers to that, how the author feels about the topic  or subject, or towards something.Whereas the 'mood', refers to the feeling of the surroundings the author is describing.The tone of a poem is the attitude you can feel about it and the writer’s perceptions toward the subject or audience. The poet uses various phrase that can tell that you should change the tone .