Understand yourself better so you can make better decisions.
Use other items as marking points.
To continue with the jam example, you might dislike a certain brand because it is too slippery. You can identify this by considering other slippery food items, such as Miracle Whip, which is a 13 on a slipperiness scale of 0-15. If the brand of jam is more slippery than Miracle Whip, then you know the reason you dislike it is because it is much too slippery.