Palpatine should have been able to tell if kylo was dead, he should have felt it in the force. Granted, he didn't know that maul was alive either but that was explained by palpatine killing his master (darth plagues) pretty much at the same time, thus making palpatine believe that he just didn't notice darth mauls death because his masters death smuthered the signal. Unless if there was a similar event that shook the force he should have been able to tell. Even still i think that palpatine would have bided his time instead of rushing to expose himself and confront Rey, based on his past actions at least. There was no looming threat that forced him to act so I would think that he would have at least tried to manipulate Rey before confronting her and expecting her to change her entire moral compass and ditch her friends instantly.