How can I tell if a field exists in a block of a matrix?
You can use {% if block.stepAccordionContent|length  %}...{% endif %} to test if any rows are defined in your table field.

Note: When you create a table field it will have an empty row defined under 'Default Values', which should be removed; otherwise all new instances of the table will have an empty row defined by default causing block.stepAccordionContent|length to return 1, instead of 0 as expected.