Hilchos Rechilus, chapter 5, section 4
30. Even if one saw no clear signs of such a thing but only surprising behavior that slightly
indicate that something is happening, one may investigate. One should not absolutely believe
what one hears.
The degree of suspicion is according to all the rules explained in hilchos lashon hara (p 61).
One may not speak against or cause any harm to the person because every Jew is presumed to be
kosher. Nor may one exempt oneself from money one owes to him even if one would be exempt
from paying him according to what one heard. One may also not hate him, and must do kindness
to him as with any Jew.