If you know the difference between em dash, en dash, and hyphen, you start seeing it everywhere—whether thet are used correctly or not. Books tend to have correct typesetting, so if you see a dash used as an em dash ought to be used, and if it looks kinda long, you can assume it's an em dash. AFAIK often manuscripts are submitted either with hyphens or --- in place of em dashes and then the editor or typesetter fixes it.

Also, it's called em dash because it's as long as the letter m (as a rule of thumb), so it's usually an easy visual comparison. Finally, a typeface with hyphens as long as em dashes would be terrible and quite noticeably wrong!