Do they resemble the shape of them being wrapped up like the other flukes? Do you ever find a symmetrical hole in the body of the casing? These are some other signs to look for. Also do you find strands of material attached to the inside of the skin if you can tell. These are things that I look for. For me, the majority of flukes have a rough texture to them. I have some now that are a different color, more brownish without the texture but the symmetrical hole in the middle of the body and they look the same rolled up as the others. So, it can be hard to tell.