In Big little lies S02E03, Mary Louise asks Jane to have Ziggy take a paternity test. Jane declines and then Mary Louise shows up at her parking lot.
We then see Jane getting in her car from somewhere and being upset. Does this mean she took Ziggy's sample or something to a lab? We later see her in a meeting with Mary Louise where she is showing Jane her son's childhood photos that look like Ziggy, meaning she's convinced they look like each other.
I cannot say if she was convinced only by seeing how Ziggy looked, or she had first gotten lab results as she was inclined not to believe that. Also, I couldn't find a reference in the episode to Jane's getting in her car upset so I connected it to her giving in to Mary Louise's request.