Please see http://www.astrostitch.co.uk/plant.htm.

Leon Fisk schreef
So how can you tell it is Lunaria annua and not Hesperis
matronalis (Dame's Rocket, Sweet Rocket). I always have to
wait for the seed pods to start forming to determine which
of the two it is. Curious...

+ + +
Well, I suppose the best clue would be leaf shape: shape here is pretty
typical for Lunaria annua, and rather unlike Hesperis
cf http://www.bioimages.org.uk/HTML/T51058.HTM
PvR