Preview of screen on second monitor
Using various software with a secondary display, you can enter "presenter mode" and this allows you to see the current and next slide, with notes that only the presenter can view.

This is available in Microsoft Powerpoint and Apple Keynotes. With Powerpoint, you can enter 'presenter mode' using Option-Return [⌥↩︎]:



With Keynote, you can customise presenter notes and enter presenter mode using the menu option: Play > Customise Presenter Notes.

I can't test as its the weekend but if I was at work, I could connect to a display and verify that it enters (as I believe) presenter mode automatically