pre heresy the thousand sons had a high amount of psykers and even before the heresy dabbled in daemonic sorcery... but to openly deal with daemons was still seen as less than \"kosher\" from what I have read. It was only after their primarch magnus the red (born missing a single eye...), a powerful psyker used daemonic sorcery to send a message (as opposed to astrotelepathy) to the emperor... he was viewed as a traitor... from there it kinds went down hill... and eventually lead to not only the restrictions and introduction of psykers and librarians into chapters but chaplains to watch over and make sure nothing dark becomes of a chapter... At the fight between Leman Russ (chapter master of the space wolves) and Magnus, magnus was winning but Russ nearly killed him and in his last moments magnus sealed the deal with tzeentch, and their city and the thousand sons were transported to the eye of terror...
additionally their symbol was different as well. Even pre-heresy they did have sorcerers and psykers intertwined... ... since they were not watched over as most chapters are are post heresy it leaves it kinda open... take a look here for a sorta collection of info...
http://en.wikipedia.org/wiki/Thousand_Sons