I need your suggestions about how to approach this problem

Yes, grid system would be required here, to verify the cards are placed in the correct locations

How can we control if the player placed the cards in right order? It's simple, - no need for openCV hahahah

structure:

Recall, when working with UI elements (children of Canvas), they are drawn top down

  - Canvas
 ----ImageA
 ----ImageB
 --------ImageX
 ----ImageC

The order in which unity draws them is A, B, X, C

As user places cards, parent them under canvas. Once user placed all cards, compare if it matches with intended hierarchy (A, B, X, C) etc

non-ui visualization: if you are dealing with more complex thing (non-UI), to visualize them you can also look into shader draw order. However, comparing with hierarchy structure to see if player won the game would still be a must.

As for the order problem:

Use an ortographic camera, for every card the player places increase the Z position value for the next card. After all cards are placed you can check the z values with the z values of the correct solution. Due to higher z values the card overlap will also be correct.