I find that Outlook calendar (I currently use the 2016 version) doesn’t highlight optional meeting invites very well. You have to read the fine print of the invite or dig around in the Scheduling assistant in the Calendar view.
One trick I found on stack overflow lets you change the color of any meetings on the calendar where you were an optional invite.
- Right click on the calendar
- Select View Settings
- Select Conditional formatting
- Add a new rule, give it a name and a colour of choice
- Select Condition and then Advanced
- Click Field, select All appointment fields and then Optional attendees
- Set the condition to includes and the value to your name.
Optional appointments should then show up in a different colour.
I made mine red. I don’t attend optional meetings :)