Detecting incognito mode

Hi all, just remembered that at IETF 101 PING meeting we'd had a brief
discussion about how websites can detect incognito mode. Did a quick Google
search and a stack overflow answer came up.
https://stackoverflow.com/a/27805491

Basically try requesting the file system API and if it doesn't exist then
you're in incognito mode. I'd imagine that to mitigate this one could patch
some js on so that the call to check if file system API exists returns true
but then you have an arms race where the only way to win is to reintroduce
the API entirely in incognito mode.