logic gates - How do we decide which circuit is a latch?

1. 
   

   Will A and B always be the complement of one another regardless of what logic gates we use in the latch-like circuit? For instance (the two circuits shown as examples):

   
   


2. 
   

   How do we decide whether A and B will be the complement of one another or not?

   
   


3. 
   

   How to find the invalid states?

   
   


4. 
   
   

   Are these two latches?

   
   

^{simulate this circuit – Schematic created using CircuitLab}

OR

^{simulate this circuit}

I have tried following a few videos for instance this. But the presenter says that a certain state is invalid because Q and Q' should be the complement of each other. But how do we decide/prove that Q and Q' are the complement of one another?

UPDATE:

Truth Table of First Circuit:

How do I derive answers to the above questions from here.