It is possible to test the web site before committing, full instructions available here: Testing your GitHub Pages site locally with Jekyll
If you have already installed all the dependencies, just run:
bundle exec jekyll serve --watch
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 jekyll startup:
bundle exec jekyll serve --watch --future