Screen reader testing means actually turning on a screen reader, ideally with the monitor off or the eyes closed, and navigating a site the way a blind user would: by headings, links, landmarks, and form fields, listening for whether announcements make sense and whether every interactive feature can be reached and operated.
No automated tool can substitute for this step, since a scanner can confirm an image has alt text but cannot judge whether that alt text is actually accurate or useful, and it cannot detect a custom widget that looks fine visually but announces nothing meaningful at all.