I think the simple test is as follows.
A) Determine the target of criticism/ agression/ attack:
If 1) it is an argument or a point of view, you are witnessing a lively debate.
If 2) it is a person or group of people, you are witnessing a flame war.
However part B should be noted. The above distinction is not concrete:
B) Determine the level of justification given for people's points of view.
If 1) it is thought out and reinforced/ reinforceable, you have a debate.
If 2) it is very simple and essentially the shouting of a base belief without justification or it has large amounts of unreasonable agression, get a bucket of water because you're looking at flames.