The best way to establish the distance visually is to simply draw a dimension. 

For example for two walls going horizontally, start the dimension command, in the popup dialog, select y-axis, and enter to return focus to the drawing, then move cursor near the one face and linesnap (shortcut "K") and same for the other face, and then a third point for where the text should be.

Unfortunately, a distance-between dimension is not associative, so the simple act of moving the wall does not update the dimension.

You need to manually move the endpoint of the dimension extension line and snap it back onto the line of the wall.

Best way to do that is to work in point-select mode, and just click on the end-point, and then linesnap (shortcut "K") to the new location of the wall.

If you move the wal using the "stretch" command, then the dimension will move with it and update automatically.