When you use a striped candy, you can tell which direction it will clear based on the direction of the stripes:
The direction of the stripes is determined by the direction you moved the final candy to create the striped candy, as described below.
According to my experience and online sources:
The direction of row/column destroying candy (indicated by the direction of the stripes on the candy) is determined by the direction you moved the piece to create the special candy.
-X-- Move horizontally --X- creates a row destroyer that destroys horizontally
X-XX Move vertically XXXX creates a column destroyer that destroys vertically
Moreover the location of special pieces is the location of where the piece that was moved to create the bonus ended up. So:
In the first example, it doesn't really matter because the special candy will fall to the bottom.
In the second example, the bonus would appear at the location second from the left like so:
In the event that you move a striped candy to create another striped candy, the location of the new striped candy will be randomly selected from one of the other candies in the set and the striped candy you just moved will activate.