CCK is an awesome module but has terrible, outdated documentation. :(
Looking through the module files revealed content_fields($field_name) which may provide the functionality you are looking for.
The function takes in the field name and returns an array of all the settings for that field.
The node type is stored in ['type_name'] so you could write something like the following:
$field = content_fields('field_myfield');
$node_type = $field['type_name'];
That should give you the node type for field_myfield.