Start with the supplement facts panel, not the marketing. Check the serving size and the amount per serving; a bigger bottle is not always better value once you divide by servings.
Look for third-party testing marks where available, and prefer forms your body absorbs well. If you take medication or are pregnant, treat any new supplement as a conversation to have with a professional first.
Note: statements about supplements have not been evaluated by the FDA, and these products are not intended to diagnose, treat, cure, or prevent any disease. Buy for general wellness, not as a substitute for medical care.