Oh, and I do have some info on Geodes, TimHortonsAddict.
As far as Geodes go, to my understanding they'll feel a bit lighter than a regular comparable stone, Being hollow and all. Though I'd imagine you might need to have a bit of experience to be able to sense that.
From what else I read, they'll sometimes have a cauliflower-ish shape, just oddly-shaped- not like a proper round boulder.
So look for anything out of the ordinary I guess. Leave no stone unturned?