1) check which subclass the membership is
You can do that by checking the attribute
if hasattr(membershipobj, 'clubmembership'):
#its base for ClubMembership
elif hasattr(membershipobj, 'forummembership'):
#its for ForumMembership
2) I can't access the 'club' or 'forum' field
Access child objects fields, through child object
membershipobj.clubmembership.club
membershipobj.forummembership.forum