Can you find anything that is permanent?
Can you find anything that is ultimately satisfying?
Can you find anything that is independent, separate from the many causes and conditions that came together to produce it?
Look at your internal experience (sensations, feelings, thoughts), and look at things in the world around you.
Especially look at things you really like, at things you really dislike, and at things you tend to ignore.
Look deeply, with your body, using all your senses. Use your heart and your intuition, as well as your thinking mind.
Look, again and again, putting everything that arises, and especially those experiences and events that are challenging for you, to this three-fold test.