This would be the macro to get a user's roles, it is a collection of RoleInfo objects though, Ilesh is correct if you want to do a check, but if you want to get all a user's roles, this is it.
{% SiteObjects.Roles.Where("RoleID in (Select RoleID from CMS_UserRole where UserID = '"+MembershipContext.AuthenticatedUser.UserID+"')") |(identity)GlobalAdministrator%}
Another option is to make a custom macro method that can return the user's roles, may be cleaner!.