You can't. It's as simple as that.

Unless you are a biochemist with access to a fully functional lab... you have no way of knowing.

In its purest form, cocaine HCl is hygroscopic.

Hygroscopic substances are those that absorb all the moisture it can hold from the atmosphere creating a soft, wet, paste-like substance, like a fine, powdered almond paste, a wet marzipan.

As the cocaine goes along the path of dealers, it gets cut, using different chemicals to decrease the purity of the cocaine and increase the profit margin.

Common chemicals/products used to cut cocaine include the list below (and a lot of other dangerous /cheap substances):

-Quinine

-Lactose

-Lidocaine

-fiber pills

-Laxatives

-Talcum Powder

-Caffeine

-Aspirin

-powdered vitamins, including C and the B family

-opiates

-amphetamines

After the cocaine is full of crazy products, it finally gets dried, and powdered up... since it is so far from its original form, it has lost its hygroscopic properties.

Oh, and it doesn't end here.

The next person/dealer who handles the cocaine will most probably cut it again, and so on down the line, until you are left with a very questionable sample.

So, there is no way to know if your sample is pure.