Clash-of-clans – What happens when you enter a clan war with a building that is currently being built

Whatever the highest level your buildings are before the end of the preparation day is the level of those buildings during the war. Anything under construction in your "normal" base will revert to the level it was before you started the upgrade in your "war" base.

This is from the Clan Wars FAQ, written by a SuperCell employee:

The war base can not be directly upgraded or rearranged, but will instead mimic the arrangement and highest completed upgrade levels of your village during the preparation day. Buildings and traps will never be “under construction” in your war base.

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.