The GFX/tilemap viewer can be accessed by pressing F4 during the emulation of a hardware set. This feature allows you to see the current palettes, how the graphics ROMs are decoded, and which tilemaps are currently constructed.

After pressing F4 to bring up the viewer, navigating through the different options is simple:

enter - Switch from the palette display, to the GFX display, to the Tilemap display, and back.
esc   - Exit the GFX/tilemap viewer.
p     - You can always pause or un-pause the emulation while in the GFX/tilemap viewer.

In the palette view, the following options are available:

+/- - Zoom the palette display in and out
[]  - Select which colormap is displayed
up/down, pageup/pagedown, home/end - Navigate the current colormap

In the GFX view, the following options are available:

+/- - Zoom the GFX elements in and out
[]  - Select which gfxset is displayed
 R  - Rotate the GFX elements clockwise
left/right - change which segment of the colormap is applied to the GFX elements
up/down, pageup/pagedown, home/end - Navigate the current gfxset

In the tilemap view, the following options are available:

+/- - Zoom the current tilemap in and out
[]  - Select which tilemap is displayed
 R  - Rotate the tilemap clockwise
up/down, left/right - Navigate around the current tilemap
left shift - Modifier for the arrow keys.  Hold to set step to 1.
left control - Modifier for the arrow keys.  Hold to set step to 64.