How do you find out if someone died in your apartment?

Your local library may have records if it were reported at the time. Most libraries have local newspapers available for you to scan if you have some idea of the time period.

Or ask neighbors and previous owners or renters of the house.

You could contact the local police department and ask them how to do that kind of "detective work", but only contact them through their non-emergency phone lines.

Or contact the local newspapers or TV stations and ask for an investigative reporter and see if they will talk to you about how they would go about it.

If you can't find anything about it easily, then it may not have even happened. There is no reason to be afraid even if it may be true, as long as you take ordinary precautions for home safety. It would be very unlikely that you would be a target of the same attacker. If it is ghosts or similar concerns you have, there is really no reason for you to be afraid that, either, especially if you have been living there and nothing has happened so far. That stuff is more from the plots of books and movies than from real life.