Hi,
You cannot know this purely from the API.
If you need to support both company and administrator logins, AND your integration workflow asks your users for the corresponding credentials - then you need to ask them for either 3 or 4 values anyway, in which case it'll make sense to have them make an initial, separate choice of whether to use a company or administrator login.
If you HAVE a set of credentials - agreement number (unknown type), user ID and password - and wish to check whether it's a company or administrator user, you can always figure it out by logging into the e-conomic application - since the subsequent user interface will reflect the user type

Best regards,