Verify that Docker is installed on my macOS machine?

You can always just execute the docker command in your terminal.

If docker is installed on your machine, it should output all of its commands:

Usage:  docker [OPTIONS] COMMAND

A self-sufficient runtime for containers

Options:
      --config string      Location of client config files (default "/Users/nickmajor/.docker")
  -c, --context string     Name of the context to use to connect to the daemon (overrides DOCKER_HOST env var and default context set with "docker context use")
  -D, --debug              Enable debug mode
  -H, --host list          Daemon socket(s) to connect to
  -l, --log-level string   Set the logging level ("debug"|"info"|"warn"|"error"|"fatal") (default "info")
      --tls                Use TLS; implied by --tlsverify
      --tlscacert string   Trust certs signed only by this CA (default "/Users/nickmajor/.docker/ca.pem")
      --tlscert string     Path to TLS certificate file (default "/Users/nickmajor/.docker/cert.pem")
      --tlskey string      Path to TLS key file (default "/Users/nickmajor/.docker/key.pem")
      --tlsverify          Use TLS and verify the remote
  -v, --version            Print version information and quit

. . .

If not, the output will contain an error similar to this:

-bash: docker: command not found

The which command should work for this:

which docker

If docker is installed on you machine, it should output the path to the executive:

/usr/local/bin/docker

Open the Finder file explorer GUI and open the Applications directory.

You should see Docker in that list of applications installed for your MacOS machine.