A while ago, I wrote this answer on Quora, it's been over 3 years, so I thought I'll update it.
I stand on my previous answer, one should never ask a girl what her operating system is, at least not directly. There are hints and indirect questions that could give you ideas, but please do not ask directly, it gets ugly pretty quick.
So how should you approach this?
First, see what she thinks about updates. Does she hate them? (windows) Does she get excited? (mac, Ubuntu) How frequently does she do it?
Next, see how her computer reacts to certain commands. A good one is ping. ask her to run
ping 126.96.36.199, if it ever finishes, it is windows.
Then go on and ask about Windows subsystem for Linux. If she is excited about it, she is a Windows user who is enthusiastic about Linux. If she hates it, she is a true Linux user. Neutral means either mac or windows.
Another great tactic is to look at the type of files she uses. Does she use docx format or odt? Does she uses rar, zip or tar.gz? These don't tell us about OS, but certainly tell us about freedom.
Ask her about her editor. Is she an MS Office junky or does she use something else? An emacs person is definitely a GNU/Linux user, while those who use vim might be either a Linux or a mac person.
Look at how she syncs her files, is it Nextcloud/Owncloud? Or does she use Dropbox or maybe even OneDrive? Google Drive doesn't tell us anything, unfortunately.
So, what do you think helps us find out what a girl's OS?