Ah, a shame. At least I am not alone, the problem you describe in your post is the exact same I'm having and what motivated me to ask here.
I'm assuming you have found some kind of solution of your own for this, but just in case anyone's interested:
I've built a (ugly) command chain for myself to check for such tags in my extracted EPUB files:
Code:
grep -r -n -I -P "<[^\s/]*>" | grep -P "\.xhtml" | grep -P -v "nav\.xhtml" | grep -P -v "<head>" | grep -P -v "<title>" | grep -P -v "<body>" | grep -P -v "<p>" | grep -P -v "<li>" | grep -P -v "<tbody>" | grep -P -v "<tr>" | grep -P -v "<td>" | awk '{print $0,"\n"}' | head -n -1