For code efficiency, you should always create a if statement similar to how Cosmin Popovici has it or else you will get an indexing error when the value of your custom fields is empty.

<?php

$args = array(

  "post_type" = "store",

  "posts_per_page" = 25

);

query_posts($args);

while ( have_posts() ) : the_post();

  $custom_field = get_post_meta($post->ID, "_simple_fields_fieldGroupID_1_fieldID_8_numInSet_0", true);

  if(!empty($custom_field)) {

    // do your code here

  }

endwhile;

?>