I started by removing my gear to get best controlled answer possible to make my damage not fluctuate so much. I equipped a low level bow to my demon hunter and went to hell act one. There I started my tests on zombies using my elemental arrow since it does the same damage under different element types due to runes. Against the zombies I use Elemental Arrow with no runes as a basic constant. It dealt around 200-300 damage each shot. I then changed the runes to Frost Arrow and electric Arrow. The outcome was the same 200-300 damage. I decided to test my idea on a more diverse group of enemies. I found some carrions and moon clan, repeated the experiment and had the exact same outcome.
I decided to use test on a creature that I was positive had some type of resistances, Inferno Zombies (which are on fire) in the Halls of Agony level 2 in Act 1. I shot the monsters with Fire and it averaged around 160-250 a shot. Electric averaged 200-300, and frost arrow averaged 200-300. I then decided to try to see how this scaled with gear. I equipped some of my gear and went back in to the area. My Fire arrows were averaging around 1,800 while my frost arrows were averaging around 2,800.
As A final test I decided to go to act 2 and hunt down a construct. I found a Chilling Construct and a Burning Construct and shot basic Elemental Arrow (which is fire damage) and averaged for around 2600 damage on the chilling but 800 on the burning. Swapped to the Frost arrow rune and averaged around 1000 damage on the chilling and 2400 on the burning. Then swapped to the Electric Arrow and averaged around 2500 damage on both constructs.
From the test I have gathered that an enemy has a high resistance to the element that it is, but will not have lower resistances to the opposite elements of its type. Also, an easy way to tell what element type a creature is would be by the type of damage it is dealing (example the Constructs stated above.)