Ask your real estate agent if any deaths occurred in the house before you buy. Your agent is required to tell you only about violent deaths in some states. If you ask about deaths in the house, though, it is illegal for the real estate agent to lie to you. Your agent has to tell you about any deaths he knows about, whether they were violent or not. If your agent does not know anything about the house, ask the seller.
Interview the neighbors about the house. If a mass murder occurred in the house, the neighbors probably know all about it, even if it happened before they actually lived nearby. Deaths, especially violent ones, are big news in most neighborhoods. The neighbors also have no reason to lie to you about the house.
Pull the police records for the house. You may have to pay a fee for this, but you can ask the police to print any calls made to the address. You would definitely find out about recent deaths this way, whether they were violent or not. You would also find out about other violent crimes, like rape and spousal abuse, that may have occurred in the house. As a bonus, you would learn if the house has ever been burglarized.
Google the address and see if anything comes up. If an unusual death occurred recently, it would probably show up on a Google search. Sheree Curry of Housing Watch suggests that you also search for the street and city names with the words "in the block of" so that you pick up any reports without your house's specific number.
Check the city records to see if the house had ever been torn down and rebuilt. This is a clue that something bad happened in the house and it was unsalable. Serial killer John Wayne Gacy's house sat empty for 10 years before it was torn down and rebuilt for new owners.