The short answer is to know your herbs well. However, we all can’t be experts of the 1000+ herbs available in the market. I use certain methods to ensure purity. For example

1. Buying only whole dried instead of powder. This method greatly aids in preventing adulteration at many levels. (eg: Henna, Moringa, Gudichi etc)
2. Having a rough idea of leaf structure, plant structure etc of the basic plant that you are trying to buy. For example, many a time I have noticed that people simply substitute Wedelia for Bhringraj. With a brief examination of the leaf structure you will be able to accept or reject the lot
3. Please check with the seller if they are educated on herbs. Sometimes, even with seasoned sellers, mistakes do happen. For example, there are several instances when sellers send brahmi instead of neer brahmi. In the marketplace, there is still confusion between Bacopa monnieri and Centella asiatica. I find it is better to send pictures of the plant you want, to avoid confusion.
4. If certain aspects of the herb is very important for you in your preparation, like flower color, then my advice is to buy fresh herbs with flower and then dry in shade. It is very possible for your seller to send the normal Arka instead of the white Arka and there is no means to check without the flower (Calotropis gigantea)

If your quality control measures are in place, then you will find it easier to purchase ayurvedic herbs from around the country.