Is the leader alive or not? Alive is likely a cult, dead is usually religion.

The next question is how isolated from friends and family or society at large are the members. More isolated is more likely to be a cult.

Other than that, there’s not much difference.

The usual setup is a cult is formed and then the second or third leader opens things up a bit and transitions it into just another religion… But sometimes a cult can be born from a religion as a small group breaks off to follow a charismatic leader.