I know this is an old thread about Outlook 2010, but it was the first search result addressing the same question for Outlook 2016. Informed guesses are as close as you can get if you're not the organizer and the calendars aren't shared.
If you open the occurrence you'll see the Scheduling Assistant button in the Meeting Occurrence tab.
It will show all the invitees and what you are allowed to see of their calendars. If their calendar is shared with you, you'll see their meetings. You'll just see colored and and patterned bars if their calendar is not shared. If there's no bar at the appointment time, you can be sure they have not accepted, but you can't know if they haven't responded or declined. Similarly, you can't know for sure if they're shown to be busy, if they accepted this appointment or some other event.