How can I tell if moodle cron is working?

It's true —as edelgado said— that if the cron job hasn't run since 24hours there will be a message in the notification page.

However, if you want to run immediately the script, you could just go to:

http://www.mymoodlesite.com/admin/cron.php

Where www.mymoodlesite.com is the address of your moodle instance.

The page will show the output of the script as it is run by the server. If anything works as it should it will print similar lines at the end of the page:

Cron script completed correctly
Execution took 1.938528 seconds

You need to output Moodle cron run to the logfile, e.g. your cron.d record may look like:

* * * * * www-data /usr/bin/php /var/www/moodle/admin/cli/cron.php >> /var/log/vle/moodle-cron.log 2>&1

Then, the content of /var/log/vle/moodle-cron.log will tell you immediately if cron running fine. If you want to go further, you can even monitor the file for the last update using some monoring tools and get alerts if cron has stopped.