There are a total of 4 Weapon Effects (not including None) that can be used to modify Two-Handed Weapons in the game so far. You can see the weapon effect in an Armory I or Armory II workbench. It should appear on the right side of the screen with the other weapon stats once a weapon is selected (below Weapon Type). For each gun, the weapon effect of a gun is determined by the Mag Weapon Module of the gun. Currently, there is no way to consistently determine the Weapon Effect associated with the Mag, except by placing it on a gun in the Armory Workbench. However, sometimes the effect of the mag will be included in the name of Mag.
Weapon Effects
There are currently four weapon effects. These are Toxic, Fire, Cryo, and Electricity. The damage for the effects are determined by the weapon stats.