Could you spot Johnny cheating at cards?

I couldn't spot any cheating.
I don't think they worked any actual switching in there. They probably just shot a couple of feet of him shuffling the deck in a fancy way without any actual "cheating" going on.

It would have been nice if they showed how he was cheating.

Having watched the scene very closely far more times than is probably healthy in order to determine this very thing, I think I've caught glimpses of cards being added to the deck as he's shuffling them (especially the second time) but it's almost impossible to tell for sure. There's one moment in particular where his hand blocks the camera's view of the cards just as one seems to fall from an impossible angle judging from how they'd just been shuffled, but it could just be confirmation bias on my part.

To be honest, I think Johnny's narration combined with the close-ups of the cards being cut and shuffled, the reactions of the dealer and the henchmen nabbing him for it immediately afterwards is enough to assure the audience that cheating was definitely taking place and that it was happening while he was shuffling the cards. And considering that Johnny's clearly done this exact thing several times before and is therefore quite adept at it, I don't think it is necessary for us to be able to see exactly what's going on every time he shuffles the deck. It's enough just to know that there was something going on.