There are many ways to tell the differences between gold, pyrite and chalcopyrite, up to different levels of proficiency. I will begin with the brief introduction of these three compounds.

Gold is golden yellow metal with the chemical formula Au. It is highly soft, deductible and good conductor of both heat and electricity. It is very resistant to air and water when exposed.

Pyrite, known as fool's gold, is an iron sulfide with chemical formula FeS₂. It is brittle, has brassy yellow color, which is very similar to Gold.

Chalcopyrite is a copper iron sulfide with the chemical formula CuFeS₂. It also has brassy yellow color, but tarnishes to iridescent blues, greens, yellows, etc.

I. All of these three have similar colors, so how do we discriminate between them? Let me first mention the ways without much professional knowledge.

1) Color and shine:
Although all of these three have similar colors, they do have different properties in terms of color. Gold is golden color; Pyrite is brass like shiny color; Chalcopyrite is brassy to golden yellowish.

Since gold is highly deductible, it shines in any angle if you rotate the gold sample. This is because the light can always get reflected in a proper surface in gold.

Pyrite will glisten when its surface catches the light. However, because of its fewer crystal surfaces available compared to gold, when you rotate the Pytire sample, you may only get stronger reflections in certain angle. That means that Pyrite will look differently when you rotate the sample.

Chalcopyrite: when it is exposed to air, it will form other types of oxides, hydroxides and sulfates, which is why it will tarnish to iridescent colors like blue, green, etc..

2) Ductility, malleability and hardness:
Gold is high malleable and ductile and the softest among the three. It has a hardness of 2.5-3 on the Mohs scale. Pyrite, on the other hand, is very brittle and the hardest with the hardness of 6-6.5.

If you use a pestle to grind or use a hammer to shatter those three samples, you will find the gold samples to be the softest. On the other hand, Pyrite will be the hardest and shatters into smaller pieces. You can think of gold as mud and Pyrite as glass in a simple way. Chalcopyrite will behave in between the two.

3) Crystal shape:
Because of the aforementioned properties, natural gold tends to be irregularly shapes, while the other two usually have natural edges and surfaces. Pyrites in natural form comes as cubes or multifaceted crystals. For Chalcopyrite, it is predominantly the disphenoid.

4) Density:
Gold: 19.30 g/cm³
Pyrite: ~5 g/cm³
Chalcopyrite: 4.2 g/cm³

Among the three, Gold will be the heaviest given the same volume of samples. Or in other words, given the same amount of samples (same mass), gold will occupy the least volume. Chalcopyrite will be on the opposite extreme among these three, while Pyrite stays in between.

5) Heat treatment:
Gold is very resistant to air and water, even when heated in air up to 1000C. However, both Pyrite and Chalcopyrite will react with O₂ in air, when heated. Those byproducts have different colors and thus can be used to help differentiate the three samples.

Pyrite will decompose into Fe S (Iron Sulfide) and then to elemental S at 540 Centigrades.
Chalcopyrite: it will oxidizes even without heating, forming oxides, hydroxides and sulfates.

6) Chemical response:
Gold does not react with water and a lot of other corrosive acids and bases. But this is not true for both Pyrite and Chalcopyrite. For instance, Gold is insoluble in nitric acid (HNO₃), but the other two will dissolve.

II. On the professional level, there are many ways or tools to tell the exact differences between those three. Such as the conductivity, reflective, magnetic properties, chemical properties, etc.. To briefly name a few in terms of techniques or properties:

1) Conductivity:
Gold is a good conductor of electricity while the other two are semi-metals.

2) X-ray diffraction
Since the three samples have different atomic arrangements, which results in different diffraction or reflection properties, then one common tool is the X-ray diffraction techniques. The three samples will give different X-ray patterns and can be easily discriminated by such a tool.

3) Energy dispersive X-ray spectroscopy (EDS/EDX)
Energy dispersive X-ray spectroscopy is the tool to detect the existence of chemical elements in a compound. Since the chemical formula for those three samples are quite different, they can be easily differentiated. In these three samples, element Au exists only in gold, while element Cu only exists in Chalcopyrite.