It is possible to test the web site before committing, full instructions available here: https://docs.github.com/en/github/working-with-github-pages/testing-your-github-pages-site-locally-with-jekyll
If you have aleady installed all the dependencies, just run:
bundle exec jekyll serve
and then navigate to http://localhost:4000.
Not finding the blog post you just wrote? It could be, because it's labelled for a future date (and you are preparing in advance).
If that's the case, it can be shown by adding --future to the jekill startup:
bundle exec jekyll serve --future