After you have set your destination in Garmin Nuvi GPS and before you tap on the "Go" button, you can actually view the complete route that your GPS has calculated by tapping on the "Route Info" area.
You might need some passion to wait for the calculation to complete, and it might take some seconds before the "Route Info" area slide out and appear on the screen.
A map will appear after you tap on the "Route Info" area. The route to your destination will be shown as a magenta line in the map.
You can tap on the [+] button to enlarge the map, drag the map will your finger to scroll the display area.
If you are already in the Map Screen showing your route to destination, you can also reach the above map showing the complete route by first tapping on the green bar which display the current road name on top of the screen, then tap on the "Show Map" button below the next screen.
If you don't like this calculated route, try to adjust your Avoidances setting in Tools > Settings > Navigation > Avoidances. Then try again.
If you still don't like the calculated route, you can also inform the GPS to make a detour.