It is safe to assume that at the time a web activity was logged the device had a cookie installed, yes (you cannot know if it has since been deleted). Why do you ask, specifically?
Thanks Sanford - we're trying to get some estimation of how many visitor's machines we are placing a cookie on (or conversely how many visitors are blocking cookies).
Well, unless you have something to non-cookie-dependent to compare it to (i.e. raw web server logs) you can't know the percentages, but you're doing all you can.
(Note even server logs will undercount visitors because they're thrown off by people sharing the same source IP and user-agent string.)