How to check the array values, what WP_Query has brought to me?

Question

Assuming you start with:

$custom_query_args = array( 
    'cat' => 10,
    'post_per_page' => 5
);

$custom_query = new WP_Query($custom_query_args);

Then to answer your first question, you can see what is returned (for development/debugging) via:

var_dump( $custom_query );

To output your query as a normal loop, you simply need to call a custom loop, using the have_posts() and the_post() member functions of the Query class:

// Start loop
if ( $custom_query->have_posts() ) : while ( $custom_query->have_posts() ) : $custom_query->the_post();
    // Loop output goes here
// End loop
endwhile; endif;
// Restore $post data to the default query
wp_reset_postdata();