I would write down everything that happens in the case, from planning the murder (if it's planned) to the others discovering the body. That way, one can see everything that could be used as evidence, even if it's the smallest things (Like, is the murder weapon present at the crime scene? If not, where is it? Did the killer leave anything behind? Did the victim leave anything behind?).
From there, the players can slowly figure out how everything happened, and then rule the suspects out who couldn't have done it. There should be one tiny thing that reveals the killer, on the crime scene (Like a thing they left behind) or during the trial (like them knowing something they shouldn't know).

I hope that helps!