In his book How to Read Literature Like a Professor, Thomas Foster says that if a poem looks like a square on the printed page, it’s likely a sonnet.
The miracle of the sonnet, you see, is that it is fourteen lines long and written almost always in iambic pentameter. … suffice it to say that most lines are going to have ten syllables and the others will be very close to ten. And ten syllables of English are about as long as fourteen lines are high: square.