How do I know the difference between Murder and Manslaughter in scenarios?

Firstly, we would need to establish whether the person in question had the mens rea, or a guilty mind. With murder, the mens rea is plain, you need to have intended to kill a person or cause Grievous bodily harm, whereas with Manslaughter it is not as black and white. Manslaughter is the 'recklessness' of murder, it requires you to have taken a risk, or at least known that death could've occured but not necessarily that you intended to kill someone.

For example, "Jayden cut Brody's car brakes, intending for him to be seriously injured but Brody crashed and suffered serious head injuries resulting in death" - In this, Jayden would be liable for murder as he intended to cause Grievous bodily harm by cutting the brakes.

However, if the scenario proceeded as "Jayden, a car mechanic, saw that something was wrong with Brody's brakes however did not fix them and let Brody drive off. As Brody was driving home, his brakes stopped working, resulting in him crashing and dying" - In this Jayden did not intend for Brody to die, but took a risk that if his brakes did not work, he may crash and die. Murder is an intention whereas manslaughter is a knowledge of risk.