how to check if an element in an html file exists in another html file?

Imagine that I have an HTML file as one of my WordPress blog pages (let’s call it page B). I want to write an if condition inside a tag on page B to check if a specific element exists on another web page (page A).

My question is what function should I choose to check this condition? Should I also import the HTML file of page A inside page B?
And what should I do on the server-side, if I have to?

Thanks