Which is heads and tails?
From the wiki, 


  The coin used is custom-made, the heads side sporting the group's signature twelve-legged spider, and the tails side depicting a spider's web.


An example is seen in Chapter 92, when Feitan and Nobunaga had a slight disagreement on what to do with Gon and Killua and settled it with a coin toss. Nobunaga won with a heads and the coin is then shown where the twelve-legged spider is facing up.