Pay attention to things that literally don’t line up.
We’re not talking about facts—though when they don’t line up, it can be a problem, too—but actual physical objects. Do wall-hangings line up properly? Do arrangements of paintings or furniture match the evidence found in photos or dust marks on the floor? Sure, not everyone arranges items in his or her house using a ruler and a level, but items being off from what is expected is often a sign something’s amiss. Though, you’ll have to find out for yourself whether that means a body hiding behind a wall or a stolen piece of furniture.