So I don't believe those users would show up in a standard search of your AGOL members e.g. users.search(...
But would show up in the group membership of the collaborated groups.
So possibly a standard search of all your members, then a comparison of the members within each group. Members in the groups but not in your standard user search would then likely be collaboration members.
Without comparing the lists of all users to those in the groups, you could possibly look at the difference in the lengths of the dir() of each user object or somewhat more explicitly try to access a property normally exposed in a user object from your org, which may not exist (you can't see) in the user object from the partner org such as user.orgId (in a try except as it will error otherwise).