Google fixed a loophole that allowed web sites to detect if a visitor was using Incognito mode. The fix led to two new methods that can still be used to detect when a visitor is browsing privately. Google has told BleepingComputer that they stand by their two new detection methods. The methods are described below. Detecting private browsing by measuring the speed of writing to the browser’s external storage system. Google’s Chrome 76 has been criticized for not being able to detect private browsing using Chrome’s FileSystem API.
Source: https://www.bleepingcomputer.com/news/google/google-chrome-incognito-mode-can-still-be-detected-by-these-methods/