WP – How to check if parent of page is a frontpage

I’m using static page and I would like to check if current page is under frontpage/post page.
For example:
www.example.com/blog – is set to post page in Settings->Reading
How can I check if www.example.com/blog/some_page is under blog- post page? I can’t check it by name becouse it doesn’t have to be named blog.