How can I tell which weapons are one or two handed?

One easy way to tell, I've found, is by how your character holds the weapon when equipped. In the inventory screen, if the new kid extends his arm, resting the bottom of the weapon on the ground, it is two handed. If his arm is down and the weapon is held out at a nearly forty-five degree angle, it is one handed.

If there's any remaining doubt: equip the weapon, exit the menu, face the camera and look at how you are holding it. Two handed weapons will be held in both hands, while one handed weapons will only be held in a single hand, looking much like you do in the inventory screen.