That is very difficult. This is actually one of the problems associated with peremptory challenges: to know why it is that the Crown and defence exercise their peremptory challenges. It's one of the reasons why the Batson-type challenge has been adopted in the U.S., where you try to determine if the pattern of challenges is such as to give rise to a potential or a perception of racially discriminatory use of the challenges.
It's been very difficult to get to this issue.