Load the value of an entity field type in another form
Use the method FieldDefinitionInterface::getType:


  Returns the field type.
  
  Return value
  
  (string) The field type, i.e. the id of a field type
  plugin. For example 'text'.


In your case for example:

foreach ($field_definitions as $name =&gt; $definition) {
  if ($definition-&gt;getType() == 'field_custom_example') {
    // found field $name
  }
}