What’s the best way to echo out a filter variable?

Like usual. 🙂 Works most of the time unless it ends up in some place that is not echoed to the screen by browser.

echo $email_to;

And for debug it is more informational to use var_dump(). Personally I usually use this to quickly add/remove dump to filter:

add_filter('filter','dump_filter',10, 1); // 1, or how many arguments the filter passes.

function dump_filter($input) {

    var_dump(func_get_args());

    return $input;
}