How would you determine the amount of matter in a substance?
Well, the are many ways, but usually, for a solid you can use grams or kilograms. Or for liquid, use milliliters or liters. Gases are relatively harder, you need to measure the pressure.
Of course there is so many other ways, but these are just the basic amounts. You might need a website to find for example g/cm, or precise measurements for specific substsances, but these are just the basic ones though.