Unambiguous is a non-starter unless you consider Qi to be something metaphysical, which is not a universally accepted opinion. I would liken it to unambiguously proving that you can view things from a perspective centered on you. We can stand facing each other, 4 feet apart. I can proclaim "There's something 3 feet in front of me," just to have you proclaim "but that's not unambiguous, because it's actually 1 foot in front of me." Both perspectives describe the same thing, they just approach it from different points
The next step would be to try to identify something which is so difficult to explain in a non-Qi mindset that others cannot reproduce it without Qi. However, if Qi produced something rapidly which could not be explained in any other way besides Qi, there's no reason to believe we would have such a question about Qi in modern western mindsets. The highly down-voted answer of doing zhan zhuang for 100 days is actually not wrong, despite all the downvotes. If you want something truly unambiguously uniquely Qi, it's going to take time. This is true for any martial art. You won't find anything unambiguously uniquely Brazilian Jujitsu in a hold or a joint manipulation until you take some time in that art either.
I have heard one way to feel Qi quickly, which you may judge for yourself. Stand in a doorway, and press against the doorframe with the backs of your hand, hard. Hold that for 30-60 seconds. Then step out of the doorframe and feel your hands rise. There are some who argue this is either Qi or some proto-qi. It is also naturally explained by science, but that is okay. I did point out that you're not going to find a simple example that is unambiguous.
If you could conjure the feeling of your arms rising on their own, and have them rise along with that feeling, without any real conscious effort on your own part, I think you may have felt something that can be described as Qi. However, to turn that into a full fledged claim of "this is how you can experience Qi," I would certainly seek the input of masters first. However, I think, in this very informal setting, some assumptions can be made without seeking their input... as long as you always try to deepen the meaning of whatever interpretation you find. I will be truly sad if I accidentally create a new Qi art consisting of nothing but people flapping their arms in the air mindlessly claiming they are building Qi, just by answering a question.
EDIT: as an addendum for the skeptics, consider pure mathematics: does it exist? I have spent time tutoring middle schoolers, and their arguments have strong parallels to those made by the skeptics about Qi. When I tutor a child on algebra, and give them a question like
3+x=7, and ask them to solve for
x, they look at me blankly. They might even go through some ritualistic process taught to them by their math teacher and eventually end up with the symbols
x=4 on their paper. Then they ask me "this is pointless. Nobody needs math! When will I ever use math."
I then give them one of the traditional word problems, like "I have 3 apples, I would like to have 7 apples. How many apples do I need to get?" They immediately answer "4 apples." I tell them, "see, you just did math." They retort, "no, that wasn't math. I just had to think of imaginary apples, and I kept track of how many apples I needed to grab. No math was required! Math is useless!"
This is the same stumping ground. The simple stuff can be done using math, or it can be done using their intuition. However, at the middle school level, they cannot appreciate things like why it is better to critically damp a spring-mass system (like the shock absorber on the car). They could not possibly fathom
d^2/dt^2(x)+2*zeta*omega_0*d/dt(x)+omega_0*x = 0, or why adjusting parameters to lead to
zeta = c/2*sqrt(mk) = 1 is ideal.
Worse, to them, there's no point in learning these things. You can tell them that this equation is essential for making your car ride smooth, and they just balk and say "Someone else can do that. I don't want to do that." And that's fine. They don't have to do the one example you came up with. But eventually, in this society, many individuals find this phantasmal thing called mathematics to be essential in their daily life. Alternatively, to the point the skeptics raise, many manage to live their entire life healthy and happy without needing to use mathematics after they leave the schoolroom. That's okay. Personally, I'm an Engineer. In my chosen profession, mathematics is not optional. I must believe in it.
So how do you explain to someone, who is just learning math, why it is worth learning it, when they cannot even comprehend its power until much later in their life? All you can do is give them trivial examples, which can be dealt with by other means, and hope that they connect the dots once they reach a task that is too difficult for their intuition. A very skilled teacher can find that one example that makes the subject worthwhile to each student.