Presumably, you want to test your fax machine by sending a fax from it and receiving a fax with it. Here’s how to do both:

To test the send functionality of your fax machine, send a one-page fax to 1-888-877-1655 or 1-503-200-1300.  When it’s finished transmitting, point your web browser to Fax Toy — it’s a fun web site that displays whatever is faxed to that number. Within a minute, your fax should appear on the site. Click the thumbnail to see a large version.

To test the receive functionality of your fax machine, point your web browser to FaxZero: this site lets you send faxes for free to anywhere in the United States and Canada. Within a few minutes, your fax machine should ring — hopefully, it will automatically answer and spit out the test fax.