→ View Select Lines

With this setting all connections (lines) that lead to or away from a selected block are shown in color.

If you select a single connection configured with this function, then the selected connection is highlighted in color.

Also, if you select a block configured with this function, the reference block of the selected block is shown with its frame in red.

Under Tools > Options: Screen you can set whether the connections should also be labeled. Under Tools > Options: Colors you can set the colors to use to display the connections.