How do I know if an Google App Engine instance is Standard or Flexible?

Google App Engine has two flavors:

Give a running App Engine instance, is there a way to know through the UI if it is standard or flexible? If not, what other ways of figuring out are there?

You can check it selecting the App Engine tab in the left side and the clicking on Versions.

App Engine > Versions

You will see several information about the service, among other things the runtime where it is running. Will be it Standard or Flexible.

Also note that a service can have more than 1 instance spawned. It is the service which is defined to run on the Standard or Flexible environment.