1. Use Tools ► Design Rule Check to select the "Rules To Check" tab in the left pane, then check the box in the "Batch" column for the "Un-Routed Net" row near the bottom. Click the button in the lower left corner to "Run Design Rule Check...". Note that this check is not available online. After running the DRC, error markers will show on your PCB. You can use Tools ► Reset Error Markers to remove them.
2. Reports ► Board Information check box for Routing Information ► Report button
3. PCB Filter panel ► Query: IsConnection will show all airwires (Connection Lines) on the PCB.