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

A double action only type revolver is one that can not be fired in single action mode. This means there is no external hammer that can be manually cocked prior to pulling the trigger. Some double action only revolvers have an external hammer that has had the hammer spur removed and others keep all of the components internal, and a hammer can not be seen.

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.