Listing every word in your native tongue that exists in your own vocabulary?
Even in these days of quarantine and boredom that would be a tedious task because even the most illiterate human has a wordstock in four digits at least.
And you have other issues like defining what constitutes "a word" as distinct from another word. Should 'cat' be considered a different word from 'cats'? For example.
When I was like 13 I got into that very topic, wondering how large peoples vocabularies are, and trying to figure out a feasible way to estimate a person's vocabulary size without actually having to count each word they know.
What I came up with was a kind of game:pick one page of the big fat unabridged dictionary. The tester looks at that page and reads off each word on that page of the dictionary to a test subject. And if the subject claims they know what the word means -and if the testers doubts the veracity of the claim- the tester (me) can just ask the subject (my kid sister, for example)"what does that word mean?", and since the tester has the dictionary definition of the word right in front of them then they can see if the subject is giving a correct definition of the word even if the word hadnt yet been in the tester's own wordstock.
Go through each word on that page of the dictionary. And keep score of how many words on that page the subject knows,and then, multiply that score by the number of pages in the unabridged dictionary.If your kid sister knows 5 words on that page, and the dictionary has 2000 pages then...thats ten thousand. The resulting number would be a rough guesstimate of the test subjects total vocabulary size. Or thats what my 13 year old self reasoned. And actually i probably wasnt far off. The Nielsen Ratings, and the Gallop Poll, are based on similarly tiny sample sizes, but are considered to be "statistically accurate" for some reason known to statisticians.
I did that with each family member. My elementary school sister got the lowest (but good for her age), I got a higher number, mom still higher, and dad had the highest. Even sis got some amount over ten thousand, and Dad got like 56 thousand.
So a conservative wild guess:
An eighth grade drop out probably has a vocabulary of at least around 20 thousand words. So even an unlettered adult has about seventy type written pages worth of wordstock to list. A lot.