Before examining how tonality is established in twentieth-century works, let us review how this was accomplished in traditional tonal harmony. One important element was a descending perfect-5th root movement to tonic combined with a half-step leading-tone motion, also to tonic. The tonicizing was often made more convincing by harmonic tritone by scales degrees 7 and 4 resolving stepwise to 1 and 3. Other elements were important also, such as melodic emphasis on 1, 3, and 5, melodic skips between 1 and 5, and formal considerations.
All of these elements may be present to some degree in twentieth-century music that has a tonal center, but a traditional V7 - I cadence would be exceptional. Instead, other ways have been devised to make the tonal center clear to the listener. Essentially, these methods establish tonic by assertion - that is, through the use of reiteration, return, pedal point, ostinato, accent, formal placement, register, and similar techniques to draw attention to a particular pitch class (1). When analyzing the tonality of a passage, it's important to pay attention to melodic aspects as well as harmonic ones, since melodic factors are often crucial in determining the tonality.
(1) The term "pitch class" is used to group together all pitches that have an identical except for the octave or octaves that separate them. For example, all B♯'s C's D♭♭'s belong to the same pitch class, no matter in what octave they are found. Página 15.