As google states:

In November 2019, node auto-upgrade will be enabled by default for newly created clusters and node pools. To disable it, use the --no-enable-autoupgrade flag

How can I check if an already existing cluster (or node-pool) has this currently activated?

Edit: As Adebisi states it can be found using: gcloud container clusters describe [cluster] or gcloud container node-pools describe [node-pool] --cluster=[cluster]

However the result will only contain the entry management.autoUpgrade if it is actually set to true. An alternative is using the google console web UI.