Chofetz Chaim Daily 10 Teves 6:8 Asur to believe rechilus even if it was a casual remark. See Hilchos LH chapter seven 10-14 for all laws of masiach lefi tumo. 6:9 Mutar to believe rechilus if you have circumstancial proof ONLY if a)theres no way to favorably interpret the evidence b)proof is directly related to rechilus story c)you yourself witnessed