How to check the missing pieces of a given set?

Question

I have a collection of LEGO sets, from which only the B model is assembled and the remaining pieces have been lost.

What is the easiest way to figure out which pieces are missing from these sets?

Answer 1

You can use the Compare Sets function on Rebrickable for that (if the B model is an official model or a MOC on Rebrickable).

Here an example for both official models from the 42069 set.