Use Frame tab to see the source frames.

When working with Stereo 3D project:

Left corresponds to the left frame.

Left+Right Left and Right frames are overlayed in semi transparent

Right corresponds to the right frame.

When working with 2D+Depth project:

Left corresponds to the 2D image.

Left+Depth 2D and Depth frames are overlayed in semi transparent

Depth corresponds to the Depth part of source video.