"If we allow this, where does it end?" The rhetorical question is doing heavy lifting: it invites you to imagine the worst possible destination and then treats that imagining as evidence that the first step leads there. The move is powerful because the endpoint is vivid and the intermediate steps are never examined.
The structure is a chain of claims — this leads to that, which leads to the next thing — where each link is asserted rather than supported. Its persuasive force comes from the destination, not the reasoning: a sufficiently alarming outcome makes people stop auditing the path that supposedly leads there. Often the first step is modest and defensible on its own terms, which is exactly why the argument reaches for consequences instead of engaging with it.
Here is the part most fallacy lists get wrong: slippery slope arguments are frequently sound. Legal precedent really does extend — that is how case law works. Institutional norms really do erode incrementally, and once an exception exists the argument for the next exception genuinely becomes easier. When someone can point to the mechanism driving each step — a precedent that will be cited, an incentive that will be exploited, a boundary with no principled stopping point — they have made an argument, and dismissing it as "slippery slope" is itself a lazy move.
The test is therefore mechanical rather than rhetorical. Ask the person to name each step and say what makes it follow. Fallacious slopes tend to collapse at the second or third link, where it turns out nothing compels the transition and the argument was relying on momentum. Sound slopes survive the question, because their advocate can tell you exactly why there is no natural place to stop.