So, while this can be highly flawed, how about asking the candidate? I find more often than not that if I ask the candidate how excited they are about learning stuff, they tell the truth. Especially when I probe a bit deeper, what do they want to learn, why, and, most importantly, what is their learning process.
If someone indicates they have a periodic study system in place, that's a strong indicator in the right direction. If someone tells me they have a goal they are achieving right now, that's amazing. If they can even estimate how long it will take them based on previous data, that sounds like a winner to me.
At that point, I am confident that they want to learn, and I have to assess other things, like what's their starting point, personality, etc.
I don't have a proof of their slope for learning. They could very well be lying just to get the job, but it's way better than disregarding slope because you can't prove it.