You can check your Varnish configuration with n98-magerun2
Use the command
n98-magerun2.phar config:show | grep full_page_cache
You should see something like the following where varnish is the hostname of your varnish server and port is the tcp port the varnish server is configured to use.
system/full_page_cache/caching_application - 2
system/full_page_cache/varnish/access_list - varnish
system/full_page_cache/varnish/backend_host - varnish
system/full_page_cache/varnish/backend_port - 80
system/full_page_cache/varnish/grace_period - 300
You can also confirm your env.php is configured with a varnish host
'http_cache_hosts' => [
[
'host' => 'varnish',
'port' => '80'
]
],