In clash of clans how do you tell if enemy troops can be deployed in certain area?

You can tell in two ways: During battle, and while editing your base.

Anything shaded in white/red means you/your enemies can't deploy troops there.

To trigger the boundaries (lines that show where troops can be deployed) while in your village, simply drag a building around. The area will be indicated as a white overlay, which will fade after a few seconds.
Alternatively, in the village editor - The overlay will be shown at all times.

The overlay in the village screen.

To trigger boundaries while during a battle, attempt to place a troop where you cannot place it.
A red boundary line and overlay will appear, showing you where you can't place your troops.
Note: If you attempt to place a troop in an area you can't deploy them during the scouting phase (first 30 seconds of a match), it will not skip you to the battle phase.

As everyone said, decorations don't have any protection.

I edited your screenshot and marked all areas where attacking is possible in the screenshot below, you have three unprotected areas.

Red areas are unprotected