Count the number of lines. Sonnets are not difficult to recognize; neither are their particular forms, if you know what distinguishes them. All sonnets, regardless of form, contain 14 lines.
Familiarize yourself with sonnet terminology. Shakespearean sonnets have their 14 lines broken into three "quatrains," followed by what is known as a rhyming "couplet". A quatrain is a grouping of four lines that follow a distinct rhyme scheme. Movement from one quatrain to the next is indicated by changes in this rhyming pattern. The last two lines, the couplet, rhyme with each other.
"O, none unless this miracle have might,
That in black ink my love may still shine bright."
Recognize and compare the rhyming patterns. The best way to identify a rhyme scheme is with the use of single letters. For example, in Shakespearean sonnets, the lines will rhyme as follows:
abab (first quatrain)
cdcd (second quatrain)
efef (third quatrain)
and the concluding couplet:
This means that the first quatrain's first line (a) rhymes with the third (a) line and the second (b) line with the fourth (b) line. The second and third quatrains follow the same pattern but with different rhymes (cdcd, efef). The concluding couplet's two lines (gg) rhyme with each other.
Originating in Italy, the Petrarchan sonnet, named for the 14th century poet Francesco Petrarca, is the oldest form of sonnet. This type differs from the Shakespearean but still has 14 lines.
The terminology also is different. The Petrarchan sonnet's 14 lines are split into an "octave" and a "sestet," an octave consisting of eight lines and a sestet consisting of six lines. The octave and sestet are not separated by physical space (like stanzas in other poems) but instead are distinguished by a literary device called a "volta". The volta is a change in feeling or sentiment or perspective. Usually, the octave will set forth an idea and the sestet will provide a contrary view.
In the Petrarch sonnet the octave follows this pattern:
This means the first (a), fourth (a), fifth (a) and eighth (a) lines rhyme with each other, and the second (b), third (b), sixth (b) and seventh (b) lines rhyme with each other.
The sestet rhymes as follows:
The first line (c) rhymes with the fourth (c), the second (d) with the fifth (d) and the third (e) with the sixth (e).