Don't remember if you're registered to vote? You can check by county:
Alameda | Contra Costa | Marin | Napa | San Francisco | San Mateo | Santa Clara | Solano | Sonoma
If you still have your vote-by-mail ballot, don't mail it in. Ballots need to be received by 8 p.m. on Election Day, not postmarked. This mistake acounted for about 61 percent of uncounted mailed-in ballots in 2012.
What you should do is put the ballot back in the special return envelope and fill in the required information. Make sure you sign the ballot, and that the signature matches the signature on your voter registration form. Then you can turn in the ballot to any polling place in your county on Election Day. You can also authorize someone else to turn in your ballot by filling out the authorization box on the vote-by-mail envelope.
If you lost your vote-by-mail ballot you can vote at your regular polling place, call your county elections official for more information or cast a provisional ballot.