It's not and that is what sucks. You just find out through visual effect or just when you run into immunes. You could also try it in the tavern on the dummies. Different elements show up as different colors in the damage numbers. Electric is blue, fire is red, poison is green, and physical is white. Edit: Someone pointed this out before me, but still... there it is.
I am a tower apprentice and my friend is a dps huntress that also has a max level tower squire. My Steam ID is Majadamska, and his is robinn57.