Once again everyone seems to conveniently forget that Castiel did not need to go to the trouble of finding and burning Crowley's bones in order to kill him. While it is the only way for any normal human (who doesn't have an angel blade, the demon blade or the Colt) to kill a demon, angels can kill a demon using their blade, or by smiting them if they are a Cherub with access to the power of heaven or are a powerful Seraph like Castiel is. It did not make any sense for Castiel to need to burn Crowley's bones to kill him, he could have either stabbed him with his blade or smite him. The fact that he went to the trouble of putting on such an elaborate and unnecessary display should have made them suspicious from the start.