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Testing and filtering is accomplished by the distillation itself. A very basic understanding of what comes out when is more than enough for a simple pot still and only one or two passes. Humans have been doing that since long before the concept of a molecule even existed.

If you decide to mess around with a large fractionating column though, do read up and make sure you have a full conceptual understanding of how the chemistry works. Even seemingly benign chemicals can be extremely dangerous to ingest or handle once concentrated. (For example, vinegar. Dilute acetic acid makes for a good salad dressing. Above 90%, contact will leave you with severe burns and scarring.)