LaMotte makes a low-level copper colorimetric test kit (compare a reacted solution to a chart) that claims to go down to 50ppb (0.050ppm).
Copper is an interesting element for aquatic life - it is toxic to algae at lower levels than that which is toxic to fish. It also is known to bioaccumulate in molluscs (snails) - hence its use as a snail killer - and fish and amphibians. However, it is an important trace nutrient as well.
To get more precise and to measure lower levels of copper, a much more expensive instrument/test would be needed - the LaMotte test will give you a ballpark. It would work well for finding out if a particular rock is leaching copper into the water column.
Finally, carbonates, organic matter, and some clays help remove copper from the water column through precipitation and adsorption.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.