Well Diana used to let the game away. She had no poker face. She used to virtually sit there rolling her eyes if she was pissed off with Charles. She would listen to his speeches like she was being tortured. There was some tour to the Far East, I can't remember - Korea? where she wouldn't look at him AT ALL and we were all sitting at home going "Fuuuuuuuck....."
Another time at home when she had to present him with a polo cup he went in for the obligatory peck and she whipped her head at the last minute and gave him a mouthful of hair instead and it was front page on all the tabloids the next day.
She could be quite obvious during that era, which in hindsight was a bit much as she was shagging several blokes in succession during those years. But she was suffering from fixed persecutory delusions at the time too, she was pretty unwell.
Charles looked pure miserable during those years too but since that was his general hangdog face, you could theoretically say he always looked like that. I don't remember him doing anything obvious to give the game away. But she really wanted people to know how unhappy she was.