check if volume for the secret is properly mounted.
Shell into the Container that is running in Pod.
The secret data is exposed to the Container through a Volume mounted under '/etc/secret-volume'.
In shell, list the files in the '/etc/secret-volume' directory.
$ kubectl exec -i -t secret-test-pod -- /bin/bash
.......># ls /etc/secret-volume
        password username
        # echo "$( cat /etc/secret-volume/username )"
        my-app
        # echo "$( cat /etc/secret-volume/password )"
        39528$vdg7Jb

check if env (environment variable) for the secret is properly assigned.
$ kubectl exec -i -t env-single-secret -- /bin/sh -c 'echo $SECRET_USERNAME'
backend-admin
$ kubectl exec -i -t envvars-multiple-secrets -- /bin/sh -c 'env | grep _USERNAME'
DB_USERNAME=db-admin
BACKEND_USERNAME=backend-admin

check if envFrom (environment variable) for the secret is properly assigned.
$ kubectl exec -i -t envfrom-secret -- /bin/sh -c 'echo "username: $username\npassword: $password\n"'
username: my-app
password: 39528$vdg7Jb
