Shemiras Halashon Halachos
Hilchos Lashon Hara chapter 6, section 9 These rules apply not only when a person speaks of someone doing a wronghood but also in other cases of lashon hara (such as shaming him with his forefathers' deeds or lack of wisdom in Torah or worldly matters as explained in chapters 4 and 5). The rule is that whatever is forbidden to speak is forbidden to believe.
Hilchos Lashon Hara chapter 6, section 10 Although one may not believe lashon hara, one may suspect it is true. However this only concerns protecting oneself or others from potential harm. The person spoken of retains the assumption that he is kosher and one may not even suspect him of wrongdoing. Therefore one must benefit him in every way the Torah commands for the lashon hara does not detract from his status in any way. Therefore, the poskim write one may only suspect in cases where harm to oneself or others might result (the case of others being harmed is discussed later part 2:9). Otherwise one may not even suspect it is true.