You use your sense of touch and proprioception/kinesthesia (which tells you what position your body and its parts have, and whether they are moving or not, how fast, etc). The more attention you put to these senses, the more developed they will be. Feel the tongue with the palate and the other way around.

You can add the sense of taste to that list, if you want: learning to taste your whole palate with your whole tongue and viceversa (according to clinical studies, there is taste perception on the adult palate as well, and the soft palate has some taste buds as well).  I remember when a myofunctional therapist told me to hold a sweet with my tongue being in contact with the palate. So you can do something similar. 

Also, when you do tongue exercises, the nnng or cheesy swallow, tongue sweep, etc. maintain the position and put attention to the feeling you have back there, and differentiate between that and the feeling you have when the tongue resting on the floor of the mouth. That will help you become aware and notice when your tongue is down, so you can get it up there. With time you may feel the uvula/tonsils as well as the soft palate.

Your kinesthesia helps you to know if you are engaging/activating a muscle. Pay attention to your tongue's movement while swallowing or doing myofunctional exercises. With time and practice, it will become easier to activate the posterior part of the tongue at will.