How does a poem's appearance help to determine the tone and pacing of your reading?
Whether you can use the appearance of a poem on the page as a guide to pacing depends to a great degree upon the poet and the period.
Ancient Greek poetry was normally written in scripta continua, i.e. all capital letters with no spaces between words at all. Readers were expected to mark word divisions for themselves. Hellenistic editors slowly began to insert word divsions, mark line numbers, place accents, etc. -- but this doesn't really tell us much about performance.
Early modern therough Victorian poetry usually organzied poems on the page by metrical form rather than as a guide to reading aloud. Visual variations like George Herbert's were intended as purely visual rather than as indications of pacing.
The 18th century elocutionists in their anthologies often did mark pacing and pauses; these were not created by the authors of the poems, but were intended to benefit readers of anthologies who would be performing poems in school.
In the 20th century in free verse, line breaks are often used to indicate pauses, and visual appearance to indicate how to read aloud.