How to check the missing pieces of a given set? 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?
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.