Petrocelli 1/ 8 Mirror, Mirror on the Wall

The plot is simple: two twin sisters confuse prosecutor Fox. Obviously, key witnesses like Shirley (the sweet Erica Hagen) and others can no longer say which of the sisters they saw leaving the deceased's apartment after his death. Fox's look of bewilderment when the accused's twin sister enters the court is priceless... What I don't understand is why Petrocelli, when he is about to crown an excellent job by proclaiming his client's innocence, backtracks, asks for an interval and convinces her to confess. Why did Petrocelli realize that the culprit was his client and not his twin? It is not known. Intuition through a gesture? Bit serious.