How to keep track of cards on top?

I'm making a solitaire style game - there are rows of cards, and in order to get the gameplay to work properly, I need the game to recognize which cards are on the tops of the rows, as only those cards should be removable.

However, I'm not sure how to achieve this...

It would be much easier to help you if you could share your capx or a few screenshots.

If by "on top" you mean z-order, you should use "Card-Pick Top/Bottom".

.

Or you can add an instance variable "RowPosition" to cards, set this variable to different numbers. Then you'll be able to pick top cards by comparing RowPosition=1.