In PA, the local party chairman or a candidate submits a form to the board of elections naming poll watchers and the board of elections provides a certificate that the watcher takes with them to the poll. Each candidate and each party can have one watcher for every precinct.
Being an honest Republican poll watcher in a city like Philli could get you very beat up if you don’t keep your mouth shut as to what you see. It is my understanding from past elections that many of those voting places have no poll watcher on the republican side.