You don't have to guess - you can hold the egg in your hand sideways and 99% of the time you can feel which side is 'heavier' and which is 'lighter'.
You should store them pointy side down / air cell side up because it keeps the eggs fresher, as it helps the egg retain its moisture, keeps the yolk more in the center of the white and also reduces breakage of the point which is where the white or the liquid insides of the egg would be. When you place an egg with pointy side up / air cell down the air cell will try to move to the new 'top'.
Also, if you want to hatch eggs, you need to leave the air cell at the top.