While there is no dedicated SketchUp tool actually called a '"mirror tool," two native tools inside SketchUp - with the catchy, descriptive names of Flip along and Scale - will mirror geometry.
Select something to flip.
Press the right mouse button to open the context menu.
Select one of the colored axis options.
Note: As shown in the animation above, flipping components is done on the internal, component axes. Other types of geometry will be flipped along the global axes.
Find the Scale tool in the Tools menu. The tool also may be a keyboard shortcut, like S.
Begin dragging a middle, bounding box handle either inward or out, then stop. The animation used the Ctrl toggle with Scale (Scale + Ctrl) to scale about center.
Type "-1" and press Enter.
Relevant Plugins
Mirror TIG - Mirrors geometry at plane, line, or point. Forum discussion.
RpTools, by Render Plus, has a mirror wizard. Windows, Fee