Which is heads and tails? In Hunter x Hunter the Phantom Troupe flip a coin from time to time to make decisions. 

On one side is a spider and on the other, a web.

Here is the image of the coin:



Which is heads and which is 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.