How do you know the lamb is a song of innocence and the tyger is a song of experience?

Songs of Innocence and of Experience is a work written by the English poet and artist William Blake. It was published in two phases, with the Songs of Innocence appearing first in 1789. This initial publication consisted of 19 illustrated poems. "The Lamb" is one of the poems included in this collection. In 1794, Blake published an expanded text titled Songs of Innocence and of Experience that added 26 new Poems of Experience, including "The Tyger". The easiest way to tell which poem belongs in which group is to look at the table of contents to determine where it is placed.
Thematically, the Poems of Innocence invoke childhood and children's sensibilities. These poems tend to be lighter and more optimistic, showing the forces of good in the world. Meanwhile, the Poems of Experience evoke adulthood and the darker elements of the world. "The Lamb" evokes Jesus as the "Lamb of God" and offers a vision of the divine grounded in mercy, love, and salvation. "The Tyger" offers a much darker vision of the divine as a terrifying and incomprehensible power and asks whether it is even possible to imagine the same God creating both the Lamb and the Tyger. The latter poem thus asks of the Tyger:

Did he who made the Lamb make thee?