How can you tell if a poem is in sonnet form?

The sonnet form has been changed and adapted over the years, however there are two main structures that poets use when writing sonnets.

A Petrarchan sonnet has 14 lines and is structured as an octave (8 line stanzas) and a sestet (6 line stanzas). Its rhyme scheme is usually abababab cdcdcd / cdecde.

A shakespearian sonnet is a variation of the Petrarchan, with the same total number of lines, however is structured into three quatrains (4 line stanzas) and finishes with a rhyming couplet. Its rhyme scheme is usually abab cdcd efef gg.

Sonnets traditionally address topics such as nature and love, and are usually in iambic pentamenter, giving them songlike and rythmic qualities.