Jonathan,
It isn't my settings that matter, but those of one of the members of
the group. Is there any way I can check this?
No, the user's timezone is part of their account information, not available to group moderators.
Perhaps a suggestion could be made (in beta@) to include a display of the member's timezone in their subscription page, Membership tab. (available through the Members list). Like their User Name it wouldn't be appropriate to let a group owner or moderator change it, but displaying it might be ok.
Shal
Shal Farley