When you enter the first jhana you are still in touch with your physical senses. Your eyes are closed but you can still hear, smell, feel, and taste. This is one definite indication of the first jhana, as opposed to others. You don’t fully lose thought either. Thoughts come now and again. Since you have been thinking all your life, your thoughts do not disappear all of a sudden at the attainment of the first jhana. They are like nervous habits – difficult to wipe out at once. They are one of the things that will pull you out of the jhana. You want to be able to maintain it as long as you wish.
This passage describes the moment you enter the first jhana with all the jhanic factors and qualities.
Quiet, secluded from sense pleasures, secluded from unwholesome states of mind, one enters and dwells in the first jhana, which is accomplished by applied thought and sustained thought, with rapture and happiness born of seclusion.
– Beyond Mindfulness in Plain English by Bhante Gunaratana