Are you lazy and use '-k' to curl all the time when using Let's Encrypt staging? Or worse, use snake-oil? Or even worse, use just http for 'test'?
curl -sSL https://letsencrypt.org/certs/fakelerootx1.pem > fakelerootx1.pem curl --cacert fakelerootx1.pem https://my-site-issued-with-le-staging
There, how hard was that? Now you can test that the cert was generated properly (even though its not properly signed).