Hilchos Rechilus, chapter 5, section 4

30) Even if you saw no clear signs, only strange behavior that hints a little that something is going on, you may ask about it. But do not fully believe what you hear.

How much you may suspect follows all the rules explained in the Laws of Lashon Hara (p. 61).

You may not speak against him or cause him any harm, because every Jew is assumed to be a good, honest person. You also may not keep back money you owe him, even if what you heard would mean you do not have to pay. You may not hate him, and you must still do kindness for him, as for any Jew.