what effective ways are there to debug cron

I would create a simple plugin that spits the data out into a private admin page, you basically want to use:

$cron = _get_cron_array();
$schedules = wp_get_schedules();
$tranny = get_transient('doing_cron');
//var_dump or loop over these

There are several plugins that do this that style the output to make it easier to read and organize, also make sure you don’t have anything manually hitting the wp-cron.php (like a server side script).

Also read this: Are transients garbage collected?