Someone burned while alive gasps for air as the fire closes in, so soot ends up inside the mouth and nose. A body burned after death may still have curled hands and feet, but its airway stays clean. A sign that only a living, breathing body could produce tells a killing disguised by fire apart from a true death in a fire.
Flies are drawn to traces of blood and flesh that the eye cannot see, so a weapon that was wiped clean still gives itself away. Lay out every suspect's tool and see which one the flies gather on. Insects also choose particular openings and wounds on a body, and they arrive on a timetable, so where and when they appear is evidence too.
To find out what made a wound, try different candidate blades on an animal carcass and compare the cuts with the injury. This narrows the weapon down to one kind of tool, and that narrows the suspects to the people who own one. A guess about the weapon becomes a check anyone can repeat.
A wrongful verdict usually starts with a careless look at the dead. Examine the body closely, and adjust for whether it was buried, how far it has decayed, or whether only bones are left. Then reason from the injuries and the scene to the cause of death, so that murder, suicide, accident and natural death can be told apart.