Write a haiku-w detector
JavaScript (ES6), 73 72 64 63 54 42 39 bytes

Thanks to Neil for saving 13 bytes

a=&gt;a.map(b=&gt;b.split` `.length)=='5,7,5'


Explanation

This is a fat-arrow function that takes an array of strings as argument. It replaces each line by its word count. If it is a haiku-w, a now contains an array of a five, a seven and a five again. Since JavaScript doesn't allow us to compare 2 arrays at the same time, the array is converted to a string first, and then compared. This results in a boolean that is returned.