How to know if docker is already logged in to a docker registry server

Edit 2020

Referring back to the (closed) github issue, where it is pointed out, there is no actual session or state;

docker login actually isn’t creating any sort of persistent session, it is only storing the user’s credentials on disk so that when authentication is required it can read them to login

As others have pointed out, an auths entry/node is added to the ~/.docker/config.json file (this also works for private registries) after you succesfully login:

{
    "auths": {
            "https://index.docker.io/v1/": {}
    },
    ...

When logging out, this entry is then removed:

$ docker logout
Removing login credentials for https://index.docker.io/v1/

Content of docker config.json after:

{
    "auths": {},
    ...

This file can be parsed by your script or code to check your login status.

Alternative method (re-login)

You can login to docker with docker login <repository>

$ docker login
Login with your Docker ID to push and pull images from Docker Hub. If 
you don't have a Docker ID, head over to https://hub.docker.com to 
create one.
Username:

If you are already logged in, the prompt will look like:

$ docker login
Login with your Docker ID to push and pull images from Docker Hub. If 
you don't have a Docker ID, head over to https://hub.docker.com to 
create one.
Username (myusername):        # <-- "myusername"

For the original explanation for the ~/.docker/config.json, check question: how can I tell if I’m logged into a private docker registry