How to test my fax machine

It's not a good idea to find random fax number and send spam fax to it.
There is legal and moral way:
Send a one page black and white text fax to 1-888-hpfaxme (1-888-473-2963).
When your fax is received by the HP Test Fax Service, we will generate a return fax to you within five minutes confirming that we received your fax.
This verifies that you can both send and receive a fax with your fax machine.