How to Preview a Window Before You Open It

Method 1 of 3:

Hover over the icon of the program you want to preview on the taskbar, which will expose a window showing thumbnails of all of the windows open in that program.
Hover over the thumbnail of the window you want to preview. The rest of your desktop will fade out, exposing the contents of the window.
Return to the window you were working in before. Move your mouse cursor away from the thumbnail window.

Method 2 of 3:

Show Windows Switcher. Press and hold the Alt key then tap the Tab ↹ key once at the same time to show the Windows Switcher window. Continue holding the Alt key.
Preview the window. Using the Tab ↹ key on your keyboard, cycle through the Windows Switcher window until the window you want to preview is selected. The rest of your desktop will fade out, exposing the contents of the window.
Return to the window you were working in before. Cycle back to the window you were working in before by tapping Tab ↹ then release the Alt key.

Method 3 of 3:

Show Flip 3D. Hold the ⊞ Win key and tap the Tab ↹ key once at the same time. Continue holding the ⊞ Win key.
Preview the window. Tapping the Tab ↹ key to advance one window, advance until the window you want to preview is displayed on the screen.
Return to the window you were working in before. Use the Tab ↹ key to navigate back to the window you were working in before and release the ⊞ Win key.