How do you know if someone committed suicide or if they were murdered?

Unfortunately, even the best forensic science can't always rule out the possibility that a death is a suicide. You cannot know, for example, if a person who dies driving into an abutment at night fell asleep or did it deliberately. But forensics is very often able to detect a death that is likely to have been caused by someone else. Sometimes killers will try to stage a shooting as a suicide, for example, but there will always be some detail overlooked by the shooter that will expose the scene as staged. Things like spatter patterns, skin burns, gun powder residue, position of the body, etc. will tell a different story from that given by the shooter.