I always thought that the anonymity services you find online might not be so anonymous (i.e., Tor). Sure, a lot of them are good, and sure, a lot of people use some of them. But just because a million people believe a lie, they don’t make it true. Just because millions use anonymity services doesn’t mean you’re truly anonymous.
This article which I found on Light Blue Touchpaper, is a fascinating dive into the intricacies of such a method to find out whether someone is using such a service. Specifically, by using the heat of the quartz crystal, using the ‘clock skew’ as he labels it, you can tell.