You should read this to understand the quirks of the place_component() method:
With that observer yes. But you are the one initiating the move, so you do not need to know that a move occurs (you’re in the middle of a move.)
And just for info: component instances are a Entity subclass, so you can attach an Entities observer to them, and detect a callback to onChangeEntity(). But this will not help you, as you need to attach to the instance after you place it, and when you do, well you know the move is over.
See this example I just cobbled together:
And reference these other 13 topics mentioning place_component():