Warm three tabs of butter to cool room temperature. Different brands, ideally different styles.
Taste them plain, on a piece of neutral bread. Notice color first — grass-fed is yellow, grain-fed is pale.
Cultured butter tastes tangy and complex. Sweet cream tastes clean and simple. Neither is better; they're tools.
European butters usually have higher fat (82–85%) than American (80%). That extra fat is why croissants made with European butter are noticeably flakier.
Do this once and you'll never buy butter thoughtlessly again.