CaliAnon here. I picked up my prop 50 ballot today. Given the stories I've seen about votes being able to be seen from outside the envelope I checked and yes in Mendocino county it is possible for the vote to be seen without opening the outer envelope. Also the fold line in the ballot falls right in between the "Yes" and "No" boxes so when placed in the envelope you would only need to look at the bottom on one side (bottom front edge of envelope in my case) to tell what the vote is. But here's the kicker, that is only true if you put the ballot in the envelope in the "traditional" manner. So I did a few tests and figured out that if you put the ballot in the envelope with the folded edge up and your vote on the back, i.e. flap, side of the envelope your choice cannot be seen as a person is then trying to look through two layers of paper.
I hope the above helps anyone concerned about making sure their vote is secure, which ever way you intend to vote.
Personally I hope prop 50 goes down in a giant fireball with fireworks and lightning bolts shooting out of it, and a huge mushroom cloud over the top. But that's just me.