What is a single action only revolver? What visual clues might you have with this type of action?

A single action only type revolver is one that can not be fired unless the hammer is manually

cocked. Usually, the visual cue would be how the firearm is loaded. Single action revolvers can open or be loaded a few different ways with the standard being the side loading gate. Another way would be the breach load where the frame opens and swivels forward. You don’t typically see single actions with a cylinder release like on the double action.

In general there are less connection points in a single action revolver which means fewer moving parts to function. In most cases the trigger links directly to the ledges on the hammer when double action revolvers use additional parts. Also, in some cases but not all, a single action revolver uses a metal bar as a trigger spring when actual springs are used in most double action revolvers.