[UPDATE 08/25/2015: Over the past week or so I’ve noticed that Google Analytics is now reporting Windows 10 and Edge browser traffic! Whoohoo!]
The quick answer is…you can’t (yet). At least not easily, or cleanly.
But, there are a couple of thing things you can do.
1) Look for the new Microsoft Edge user agent string, which will look something like:
Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.135 Safari/537.36 Edge/12.10240
Mozilla/5.0 (Windows NT 10.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.135 Safari/537.36 Edge/12.10136
Note: The last numbers, the part after Edge/12, will vary.
2) Compare the Windows NT traffic you were getting before Windows 10 launched to what you see now. Any increase could be signs of Windows 10 users, who are most probably using MS Edge.
3) Grab or create a custom segment. For example: http://tinyurl.com/ngua99w
4) Wait a bit for Google to catch up. (Although, I really don’t understand why they don’t already have this in place. You’d think that they would have been working with the early releases, right?)
Here are a couple of interesting articles to watch - they’ll probably update as new info comes in: