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In what acid would a combination of gold and copper only dissolve the copper, but leave the gold?
I've been googling around, and so far they follow very close to the same solubility rules - as in, it would react with both the copper and gold, and both were semi soluble. If the compound is brightly colored that would help too.
EDIT: as pointed out by Stian Yttervik, my previous answer (below) is probably too dangerous for someone without prior experience with handling hazardous chemicals.
I would therefore recommend that you use nitric acid: simply add a few drops of clear fuming nitric acid to a test tube containing your gold leaf and stir with a glass rod.
The solution will then go green/blue if it contains copper. Only do this with very small quantities of gold leaf and when outside as one of the products of the reaction with copper is nitrogen dioxide - an extremely toxic brown gas.
I would recommend concentrated $\ce{H2SO4}$, as it is easily available and produces a brightly coloured salt when reacted with copper, thus making it easy to tell whether your gold leaf is 24K or not without any expensive weighing equipment.
Make sure you do this outside or in a very well-ventilated area, as it will give off small amounts of $\ce{SO2}$, a toxic gas. Wear thick gloves and eye protection and make sure you have a water supply nearby in case of an accident.
Pour 10 ml of concentrated sulfuric acid (98%) into a boiling tube (or any other narrow Pyrex container you might have) and heat the liquid with a Bunsen burner until it starts to boil. Avoid the fumes as these are corrosive. Without removing the boiling tube from the flame, add a few small sheets of gold leaf and stir the solution with a glass rod for a few minutes.
If you find that the solution is completely colourless, there certainly was no copper in the gold leaf. However, even the slightest tinge of blue/green would indicate copper sulfate - formed by reacting copper with sulfuric acid.
When you're done, make sure to dispose of the sulfuric acid by emptying it into a litre or so of water (acid in water, not the other way around) and then neutralising this with bicarbonate of soda
