How do we verify "npm login" succeeded and the token has not expired?

You might use npm whoami command.

$> npm whoami
${username}
$>  npm logout
 npm whoami
npm ERR! code ENEEDAUTH
npm ERR! need auth This command requires you to be logged in.
npm ERR! need auth You need to authorize this machine using `npm adduser`

npm ERR! A complete log of this run can be found in:
npm ERR!     /xxxxx/.npm/_logs/2019-02-06T10_21_10_780Z-debug.logged

In the context of GitHub Packages: if you logged in using npm login --registry=https://npm.pkg.github.com, you can use npm whoami --registry=https://npm.pkg.github.com (same registry) to verify the currently logged in user.

$ npm login --registry=https://npm.pkg.github.com
Username: <your-user>
Password: <your-password-or-your-personal-access-token>
Email: (this IS public) <[email protected]>
Logged in as <your-user> on https://npm.pkg.github.com/.

$ npm whoami --registry=https://npm.pkg.github.com
<your-user>

If I only run npm whoami I get the error mentioned in the question.

I'm posting the workaround I came up with, but I'd love a better solution.

I've got Jenkins running this bash script periodically to test/verify my npm login against a private registry:

#/bin/bash

# Suppress commands (Jenkins turns this on)
set +x

# Suppress color codes from NPM output (for proper grepping)
export TERM=dumb

# Stop on any error
set -e

NPM_USERNAME=...
NPM_PASSWORD=...
NPM_URL=...
NPM_EMAIL=...
WORKSPACE=... (in my case, set by Jenkins)

echo "========"
echo "Looking for previous failed login (cached credentials)..."
echo ""
# NOTE: A previous failed login can result in an ".npmrc" containing
# a username/password in lieu of an auth token. We look for this and
# remove it (logout) if it exists so that the cached credentials are
# not applied when we run "expect" to login
# (which would see different prompts from cached credentials).

# Chop off "http:"/"https:" prefix from URL
NPM_REPO_PREFIX=`sed -e 's~https\{0,1\}:\(.*\)~\1~' <<< "$NPM_URL"`

# NOTE: piping to /dev/null so the password isn't printed
set +e
grep -F "${NPM_REPO_PREFIX}:_password=" ~/.npmrc > /dev/null
GREP_EXIT="$?"
set -e

if [[ "$GREP_EXIT" == "0" ]]; then
    echo "========"
    echo "Logging out of repo..."
    echo ""


    npm logout --registry "$NPM_URL"
fi

echo "========"
echo "Logging into repo..."
echo ""

(/usr/bin/expect <<EOF
set timeout 10
spawn npm login --verbose --registry "$NPM_URL"
match_max 100000

expect "Username"
send "$NPM_USERNAME\r"

expect "Password"
send "$NPM_PASSWORD\r"

expect "Email"
send "$NPM_EMAIL\r"

expect {
   timeout      exit 1
   expect eof
}

EOF
) | tee "$WORKSPACE/npm-login.out"

echo "========"
echo "Verifying output of login..."
echo ""
# NOTE: If the login fails, the npm command still exits with status "0",
# so we read the verbose output to see that the http server confirms
# successful with "http 201".

set +e
grep "npm http 201" "$WORKSPACE/npm-login.out"
GREP_EXIT="$?"
set -e

if [[ "$GREP_EXIT" != "0" ]]; then
  >&2 echo "========"
  >&2 echo "ERROR: Failed to login to repo [$NPM_REPO]"
  exit 1
else
  echo "========"
  echo "SUCCESS: Logged into [$NPM_REPO]"
fi