Shulchan Arukh, Orach Chayim 214 says
כל ברכה שאין בה הזכרת שם ומלכות אינה ברכה
Any blessing which does not mention Hashem's name and Kingship is not a blessing.
If a prayer leader in a blessing does not enunciate מלך העולם are there sources that would justify me in not answering אמן?