Pull requests events by default check out the merge commit ref. Alternatively, the pull request head can be checkout using:

- uses: actions/checkout@v2
  with:
    ref: ${{ github.event.pull_request.head.sha }}

However, in successive actions it is not possible to detect whether the action uses another ref. github.context.sha still references the merge commit. See for example the discussion on: jwgmeligmeyling/pmd-github-action#2

It would be nice if actions/checkout@v2 somehow made the checkout ref available as output or exported variable.

Or am I overlooking an existing approach to obtain the checked out commit SHA?