Wordpress - How do i know the current post type when on post.php in admin?

add_action( 'admin_init', 'do_something_152677' );
function do_something_152677 () {
    // Global object containing current admin page
    global $pagenow;

    // If current page is post.php and post isset than query for its post type 
    // if the post type is 'event' do something
    if ( 'post.php' === $pagenow && isset($_GET['post']) && 'post' === get_post_type( $_GET['post'] ) )
        // Do something
    }
}

I am going to expand on MiCc83's answer. There are a few things that don't follow the OP's original questions but overall it's a great solution. For example, it would not work with a post_type event because you are checking the post_type as 'post' in the answer.

add_action( 'admin_init', 'do_something_152677' );
function do_something_152677 () {
    // Global object containing current admin page
    global $pagenow;

    // If current page is post.php and post isset than query for its post type 
    if ( 'post.php' === $pagenow && isset($_GET['post']) ){
        $post_id = $_GET['post'];

        // Do something with $post_id. For example, you can get the full post object:
        $post = get_post($post_id);

    }
}

The condition 'post' === get_post_type( $_GET['post'] ) in the previous answer would prevent this from working on a post type 'event'. You would need to check for post type 'event' instead of 'post'.