A thread on this topic was created just a few days ago.
www.elitefourum.com/t/wotc-shadowless-base-set-theme-decks/16320/1
Short answer - There is no known way to guarantee that a theme deck is shadowless (unless it has a gold strip around the top).
However, you can tell if a 2 player starter set is shadowless by following this guide.
www.elitefourum.com/t/english-base-set-2-player-starter-deck-guide/14698/1