How can one differentiate between an idea that contradicts Islamic creed and an idea for which there is no evidence to deny or affirm it?

Distinguishing ideas that contradict the sound creed is done by knowing the creed itself. The intellect is a perceiver, not a ruler; therefore, legal rulings are not derived from the dictates of the intellect, and not everything in the Shari'ah has its wisdom comprehended by the intellect. Custom is even less likely to be used as a basis for judgment, so whatever has not been proven by the Shari'ah to contradict the creed is not considered as such.