We have lots of heuristics to tell whether a predicate is vague. If it admits of borderline cases, or if it is Sorites susceptible, it’s probably vague. But predicates aren’t the only vague terms. Predicate modifiers, like adverbs, can be vague. How should we test adverbs for vagueness?For concreteness, let’s focus on a particular (intuitively) vague adverb, ‘quickly’. How could we test/argue for the vagueness of ‘quickly’.One idea is that ‘quickly’ is vague iff for some verb V, V quickly is vague. But that won’t do, because V quickly might be vague in virtue of V being vague.Here’s a Saturday night live guess at what we might do. If we can find a verb V such that there’s a sorites sequence for V quickly such that each term is a clear instance of V, then quickly is vague. That condition seems necessary and sufficient at first pass. Or so it seems to me. Counterexamples and counterproposals welcome!