This function will get you the object_index of any parent that has been assigned to the specified object, or else return -100 to show that the object has no parent assigned to it, or -1 if the object being checked does not exist. For more information on parents see the section on the Object Editor.
|obj||The index of the object to check|
par = object_get_parent(object_index);
The above code will get the parent of the object index for the instance running the code and store the return value in the variable "par".