I think using common sense and erring on the side of caution (i.e. "playing it safe") probably really is all you can do. As Elsa said, she and other staff can move/rearrange topics at will, so if a thread is in the wrong place it can be swapped in or out.
Topics that are liable to get heated or controversial generally fall into the following categories:
Topics where beliefs are potentially questioned, e.g. Religion topics, Spiritual vs Psychological topics.
Topics about social issues and minority groups, e.g. LGBT issues topics, Racism topics
Topics about very personal and possibly upsetting issues, e.g. Mental health topics, Abuse topics
Any one of those topics could potentially start out in another forum area as a more benign discussion but end up being swapped into the Heated Debates forum if it started to become too intense, or, if the objective of the thread in the first place was to generate debate or play devil's advocate about a sensitive issue, then that kind of thing would most likely be placed directly in the debate forum to begin with.
I don't think there's any golden rule or mathematical formula that could determine exactly which topics belong in there and which are fine outside, but I think we could use our best judgement and if we're wrong it really isn't hard for an admin to move the topic into the right place.