What's your use case?
Example: Amazon does not use a tracking cookie, however we can see clicks happening by analyzing our spider's backtrace to see if an Amazon click-tracking URL was fetched.
Has our spider seen your click-tracking cookie?
Example: A new ad platform wants to see which sites currently use Taboola. Additionally, you are able to see which websites have used Taboola in the past, but no longer do.
Example: A commerical forum vendor, vBulletin wants to know all the sites using vBulletin so they can compare it against their licensed customers.
Example: WebTrends wants to see all the domains that have the WebTrends analytics system installed and in-use.