From a meetup member:
Attached is a Poll Watcher Certificate that I made up.
Since there appears to be no formal organizational setup in NY, I
believe that you should be able to pass these out with your signature.
I doubt if any election worker would challenge it.
Some campaigns compare the counts on the machine at opening time and
closing time. The most important is closing time. Have poll watchers
bring flashlights to personally inspect the back of the machines as
they hold a sample ballot to record the results. Then as the results
are published by the Board, they can be checked for accuracy.
Here is additional documentation: