Which address bits are used for set selection/tag check in a cache? If 


  Capacity = 24 KiB Associativity = 6 Line Size = 32 B


As far as I know:

$$Capacity = Associtivity * Line Size * Sets$$

so
$24576 = 32 * 6 * Sets$

$Sets = 128 = 2^7$

thus 7 least significant bits are needed to identify set number.

Am I doing it correctly?
Your result is almost correct.
The number of sets is indeed 128. 
But 128=2^7
And 7 bits are required to identify a set...