Labels on a standardized product state the content of active constituents versus drug concentration ratio, assuring a consistent dose. Standardized products in any form are your best bet. The next thing to look for is how fresh the herb is. The fresher the herb, the better. Light, moisture and air rob herbs of their effectiveness.
Tinctures are another good choice, which does a good job in preserving the plant.
Skip the loose herbs sold in bulk, they are likely to have lost all their medicinal properties. Be sure you have received a correct diagnosis from a conventional doctor before seeking alternative care. Proper medical care is critical to good health. These job titles are not regulated. Work with your doctor.