I'll summon all my moral courage and provide this controversial answer: the test that rules out free will is whether we can predict the behavior of the subject. If we cannot, then free will may be present. We can strengthen this conclusion inductively to the point of practical certainty, as is always the case with induction.
(Per my kind first commenter's recommendation below, I'm adding some references in parentheses. It will take a bit to complete these edits, please be patient!)
To explain, please permit me to start from basics. We disprove assertions using the contrapositive, we prove assertions, always provisionally, using induction. One of the surest conclusions of science, inductively demonstrated a thousand times a day across the globe and through the heavens, is that natural processes are predictable and amenable to representation by "physical law". We can refine those laws to predict outcomes of natural processes to 1 part in 1012 or better. (Hoyle's C12 resonance prediction based on physical processes was good to about 1 part in 1012; multiple physical processes yield QED's fine-structure alpha values consistent to about 1:109, and we have G values across many physical processes consistent to about 1:103, the black sheep in the fold [Nature (2014) doi:10.1038/nature13433]. See also A physicists guide to skepticism, p. 102ff.) Never in any of these predictions do the phenomena ever require us to invent the concept of "free will" to understand them. Determinism reigns. Predictions are not only possible, they typically are of amazing precision and ongoing validity. We send probes to Saturn on the strength of these laws.
Under naturalism's view we human beings are no more nor less than natural phenomena. (see esp. section 1.3 in the linked page.) Yet somehow we have been required to invent the concept of "free will" to explain the behavior of living things. The clear reason for this is that living things behave unpredictably. In the case of human beings, the behavior is so unpredictable that we struggle to maintain civil society and even risk self-annihilation. (While there are many references that represent habits as predictable behavior, I find none so far that accurately represent the validity and unpredictability of arbitrary choice by humans. So for a reference accessible to anyone, I recommend you simply predict what your friend or spouse will say first at any given point in time. If you're not successful 100% of the time, just go find someone else, or some computer algorithm, that can do so with 100% success. If you can't find such a person or thing, this should clearly show that we humans are routinely capable of unpredictable behavior in utter contrast to the predictability of nonliving phenomena. You may distinguish between practical, positivist unpredictability and unpredictability in principle as I discuss below, but Occam's Razor will not be your friend, I think.) So the simple test for free will can be, "Construct a predictive law that works with a comparable degree of success as the laws of physics to predict the behavior of the subject in question." If this cannot be done, then the factor we call "free will" could be in operation.
The mainstream response to this point is typically to vigorously restate the assertion that we living things are simply natural objects governed completely by deterministic physical law, and that is that. The only reason we can't predict the precise behavior of living things, opponents will say, is that living things are the most complicated natural structures in existence, which means the problem is just too hard. But someday we will be able to make such predictions.
I would argue in reply that a truly scientific approach to this question means that in the complete lack of any evidence that such predictions can be made (that is, no one can ever predict what another person will do next in the way one can predict where Saturn will be next) we will refuse to make unsupported assumptions. If you cannot predict precisely what I and others will do next, you can tell me all day long that my behavior is predictable with the utmost precision because all of me is completely determined by natural law, but I have no reason to believe you from a scientific point of view. You may try to convince me using logical inference in the absence of any supporting data for the premise, but we normally consider arguments like that to be of the weakest kind.
So the test would be, "Is the subject's behavior predictable to a degree which matches other nonliving physical phenomena?" If so, then free will is not a factor. If this prediction is not achievable (and Libet's work does not achieve it, by the way) then free will must be considered. Moving from this to a firm conclusion that free will exists in the world then becomes a matter of inductive proof.
Please reply kindly.