If the user has selected the option ' Allow others to view my Online / Offline status from the Web.' you can see it through the ICQ Web Pages, or by using another ICQ account. (This is only possible if the user has added you to her/his invisible list, it won't work if the user is in invisible mode - I mean if she or he is invisible to all contact list)
I've found this: http://www.ifud.ru/index.php?page=dfm
But I haven't tried cause my ICQ doesn't connect nowadays...
I don't think there is another way for detecting it, there were some in the past but they are for older (very old) ICQ versions and don't work anymore.
"We see only what we know."