The only answer I can come up with is that it has to do with the scroll. Realistically, an animal would have to see Madara find the loophole in Edo Tensei and somehow travel to Jugo in that short period of time between Itachi's departure and their visit with Sasuke. Unless they have telepathy power.. not possible. Unless he was seen talking to an animal on panel or has telepathy powers that we don't know about...
Maybe Madara DOES have the curse mark and Jugo can sense this. I dunno yet