There are the IP's from North Korea available as well, it was discussed couple of time ago on HN[0]. I think checking by IP is more reliable than checking the browser: Red Star OS is not the only OS in North Korea and someone could use Naenara Browser from the outside as well...

[0] https://news.ycombinator.com/item?id=8777226

Notice that the user agent string gets rewritten to something innocuous in HTTP headers but not JS. So sniff client-side through JS.