Varnish PHP Cache Test can be done with a simple PHP Script.
For testing, we will use a random generator script that will generate random numbers between 1-99 on each refresh/reload from the browser.

Test script: varnish-test.php

echo rand(1, 99);

* You would need to place the script in your domain folder and then open it from the web browser.
example: https://domain.tld/varnish-test.php

If the results return on each reload/refresh the same number then varnish cache works fine.
If the results return on each reload/refresh different number then varnish is not caching results.

please note that the script must have proper file permissions owned by the user and file permissions 644.