To determine a poem's tone, you must identify how the speaker perceives his audience. You might first think the speaker is addressing only the implied audience -- the person who is reading the poem -- however, in "Preludes" you will find that the speaker is also addressing a particular person in the poem who is “[s]itting along the bed’s edge” and playing with the papers she is using to curl her hair. Stanza 3 makes explicit that the intended audience of the poem is this particular woman by using the vocative “you.” Thus, the tone of the poem is determined not only by what the speaker thinks of the implied audience, but also by what he thinks of this woman.
Who is the mysterious woman addressed by the poem's speaker? Judging by Eliot’s word choice of “crept,” “gutters,” “yellow soles” and “soiled hands,” you can guess that the tone of the third stanza is one of vitriol and disgust. The mystery woman may be a prostitute or someone of ill repute. Or, she could be a perfectly decent lover who has rejected the speaker, and perhaps this poem is the speaker’s way of lashing out at her.
Stanza 1 is full of vivid images that describe a particular scene: It is 6 o’clock on a winter night, and the woman does not seem to be doing anything other than observing what’s around her. The speaker chooses to emphasize dead leaves and newspapers being blown by the wind and circling around the woman's feet, suggesting a tone of perhaps pity.
After you have spent some time reading and analyzing the poem, you can then identify the main subject matter. You could argue that the main subject matter in "Preludes" is a sustained meditation on the nature of the world. So how does the speaker feel about it? The speaker seems to be given to making morbid predictions: “The conscience of a blackened street / Impatient to assume the world.” He is disgusted by the hypocritical nature of the world’s inhabitants: “And short square fingers stuffing pipes, / And evening newspapers, and eyes / Assured of certain certainties.” He may even derive a sadistic pleasure from observing these things: “I am moved by fantasies […] of some infinitely gentle / Infinitely suffering thing.” All of these words -- morbid, disgusted, sadistic -- can describe the tone of the poem.