How do I know if newborn still hungry?


My baby was born earlier this week (the 13th) and he feeds well. He latches and eats a while but I can never tell when he is actually full and when he is using me for comfort. Tonight he nursed about 20 minutes on each side before becoming super tired and floppy armed. We changed his diaper I swaddled him and now he’s laying in his bassinet turning his head side to side like he’s looking for a nipple again. But h fed so long and got so tired! I don’t want him to not eat when he’s hungry though. Any ideas in differentiating!?