I am conducting a survey, within this group, all the questions are optional. So the default answer is "No Answer" or something like that.

I just want to make sure that the participant did not accidentally skipped the question. Thus I am wondering for an optional question, is there a way to warn the participant that he/she did not answer a question and keep the question optional.

One way I envision this is, after the participant finish with a group and hit next, the survey checks to see if there are any missing answers and if there is sends a pop-up for the person saying "You did not respond to X question(s)". But if the person hits next again, they won't receive that pop-up the second time but continue on within the survey. Basically the participant was warned only once that a question was not answered.

You could pop up an alert or a confirm message with JavaScript. See this post for inspiration - www.limesurvey.org/en/forum/can-i-do-thi...-you-can-skip#123388 . You will, of course, need to modify that workaround to suit your question type(s).

Sometimes :
I use a "Multiple choice question" with only ONE subquestion:
Question "You don't answer to {count(is_empty(Q1.NAOK),is_empty(Q2.NAOK))} questions"
Sub question "Check to confirm"
Relevance : {count(is_empty(Q1.NAOK),is_empty(Q2.NAOK))}>0
This question is mandatory, then user must check if he don't answer one of the question.

Denis