human
How to tell if someone has seen your facebook event invitation?
assistant
Tell if Someone Has Seen Your Facebook Event Invitation on a PC or Mac
1. Open Facebook on a browser.
2. Click Events on the left panel.
3. Click on an event you're hosting 
4. Click on the number of Going • Maybe • Invited guests.
5. Click the INVITED tab.
6. Check for a Seen sign below your friend's name.