Raw denim is quite simply denim that hasn’t been treated after the dyeing process, giving it the uniform indigo color and “raw” appearance. Most denim jeans in the market are not considered raw denim due to the fact that they’ve been treated by manufacturers to give specific finishes desired by the brands they’re sold to.
You’ll know if a pair of jeans has been treated when you see that it has fades, marks and patterns on them despite being brand new. Rummage through the different sizes and you’ll find that the finish is more or less the same.
Given the popularity of raw denim these days, the product name and description should tell you if you’re getting raw denim or not. Otherwise, a quick Google search or query in raw denim forums should give you the answer you need.