What you are must be constant.
This is so obvious that we often overlook it, but, whatever it is that we are, it must be ever present. What we are does not come and go. An easy way to investigate this is to begin by noticing what fluctuates, what comes and goes. You can immediately verify that appearances are transient. Appearances include thoughts, bodily sensations, and emotions.
Take a moment to confirm that this is so. Thoughts only last a moment. Emotions arise and subside. Sensations appear and disappear. Notice what is present before the thought, sensation or emotion appears, and after it subsides. There is something (which is not a thing) that is constant in the presence of all of the appearances and disappearances.
Look into that which is not fluctuating. Be curious about it and really look. This kind of looking is intuitive, not thought based, and results in immediate recognition of the truth.