California Code, Elections Code - ELEC § 15104

(a) The processing of vote by mail ballot return envelopes, and the processing and counting of vote by mail ballots, shall be open to the public, both prior to and after the election.

(b) A member of the county grand jury, and at least one member each of the Republican county central committee, the Democratic county central committee, and of any other party with a candidate on the ballot, and any other interested organization, shall be permitted to observe and challenge the manner in which the vote by mail ballots are handled, from the processing of vote by mail ballot return envelopes through the counting and disposition of the ballots.

(c) The elections official shall notify vote by mail voter observers and the public at least 48 hours in advance of the dates, times, and places where vote by mail ballots will be processed and counted.

(d) Notwithstanding paragraph (2) of subdivision (b) of Section 2194, vote by mail voter observers shall be allowed sufficiently close access to enable them to observe the vote by mail ballot return envelopes and the signatures thereon and challenge whether those individuals handling vote by mail ballots are following established procedures, including all of the following:

(e) A vote by mail voter observer shall not interfere with the orderly processing of vote by mail ballot return envelopes or the processing and counting of vote by mail ballots, including the touching or handling of the ballots.