Not sure if there is a reliable way to determine if a commit is a pull request from within all CI environments, but would like to see the option explored

Use case, I don't want to deploy on Pull requests, but I don't want disable the Pull webhook in GitHub

I have all my deploy logic in a psdeply script and would like to control deploy on pull request. Reason is appveyor does not resolve secrets on PR (for public repo anyways) and thus deploy can't work. (deploy to psgallery) <- I sometimes deploy stable (-dev##) versions to psgallery

Semi-unrelated: Is there a way to tell appveyor to not build a branch if there is a pull request open (I want to avoid the double build on PR update)