1. What does it mean?
Literally asking whether something is a true story, 실화냐 is used as an informal reaction to something hard to believe. It can express admiration, frustration, or disbelief rather than request factual verification.
Also written or said as 이거 실화냐, silhwanya
2. The nuance
The ending -냐 is blunt and informal. Among friends it may be playful; directed at a stranger's account it can sound confrontational. This is not the polite way to ask whether a film is based on real events.
3. Examples
“주문한 지 십 분 만에 벌써 왔다고? 실화냐?”
It's already here, just ten minutes after we ordered? Is this for real?
“방금 저장했는데 파일이 사라진 거 실화냐.”
I just saved the file and now it's gone. Seriously?
Sources
Nate News: Korean usage in the press
References support the meaning and usage. Example sentences are written for NoCapWiki.