How to know quality of a chemical?

I understand people will say 'just buy it from a reputable seller'... That is not always the case..

I bought two carrot oil's from a two very reputable sellers, both different of course - i believed one to be purer than the other, although both claimed same % purity... However, thats not the point right now.

Let's say I'd like to buy Allantoin from a second source, a lesser known manufacturer, or any other chemical in fact - how can we know their 'stuff is good' ? 

i've been told that doing tests are limited in the lab :(
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