This probably isn't the ideal way, but a possible way to get ideas.
Chances are people will get to the site/page only if the result is in the top one or two pages of the SERPs. So then it's a matter of finding what search terms will have the webpage to show in the top 10-20 results. Then get the other pages, primarily the top few, that show up for those search terms as quality comparisons. The higher in the SERPs the page in question is, the more "statistically likely" it is to be found by users and the more relevant the comparing content.
The goal is to get any search term in which the page in question will appear in the top 10-20 search results. Rather than testing every possible search query, you can probably assume that the page will only rank for relevant terms, and only relevant terms will get users to that page. Brainstorm every possible relevant term, use Google Keyword Planner to expand the list dramatically, and Google autocomplete as well to expand the list. Once you've garnered any and every relevant keyword that the page in question could possibly rank for, then you see if that page does rank for any of those terms. Plug that list into a bulk SERP checker. A premium SEO tool might work better, but a couple free ones I found quickly http://searchenginereports.net/ and http://serp-checker.ezmlm.org/ might work. Narrow down the terms that provide the site within the top 10-20 results and those are probably the most relevant search queries. Then use the other pages that show up for those search terms as quality comparisons. The higher up in the SERP's the page in question appears, the more likely people will get to the page with that query. Thus the more relevant the competing pages are for that term and for quality comparison.
A backlink checker will also help find keywords that the page could possible rank for reasonably well in the SERP's. The anchor text used in those links will boost the SEO for those terms increasing the likelihood people would find the site with that search term. https://ahrefs.com/, https://moz.com/researchtools/ose/, https://majestic.com/
It may not be ideal but there doesn't seem to be a lot of other suggestions. Hope it points you in the right direction.