How do I discover property names? When doing a custom D7 to D8 migration, i sometimes need to migrate a specific value such as 'fid' (for example file ID of an images) into the 

'field_image/target_id'


property. How do I find out which property i can use for D8 fields? By property i mean the target_id part. For example for uid field? The devel module does not seem to be helpful here, or I may be wrong.
You can use the EntityFieldManager. This is an example how to get the properties for the field uid of the entity node:

$fields = \Drupal::service('entity_field.manager')-&gt;getFieldStorageDefinitions('node');
$properties = $fields['uid']-&gt;getPropertyDefinitions();


From the command line you can use this in a drush php-eval command:

drush ev "var_dump(\Drupal::service('entity_field.manager')-&gt;getFieldStorageDefinitions('node')['uid']-&gt;getPropertyDefinitions())"