TL;DR
Case A. There is no need to determine it inside the main template file (index.php) because it is the default template for it[1].
Case B. To determine it inside a page template (ex: page.php), simply check it like so:
get_option( 'page_for_posts' ) == get_the_ID()
Details
I literally went digging the source-code[2] of it just to be able to know how wordpress does the checking of the value. It turns out, it is using the statement get_option( 'page_for_posts' )
to know the post ID of the selected value of the Posts page.
So yeah, for this purpose, there is no such official checker function that is similar to is_front_page()
.
As long as you know the ID of the page that you've selected then you can use it for the checking process.
References
WordPress Codex, Theme Development, codex.wordpress.org/Theme_Development
Source-code of Settings › Reading Settings, github.com/WordPress/.../wp-admin/options-reading.php