Do you know any good signs that could be posted that may discourage fruit theft?
How about, "I just wasted money on this sign to deter you from stealing my fruit...please help yourself".
Sorry, seriously, why would a sign deter anybody from stealing the fruit ? They are already planning on stealing, they know the trees/property are not theirs. A sign can be not only a waste of your money but an actual invitation to almost let them know that the sign is possibly your only line of defense from guarding the theft of the fruit. If you property is fenced and you have a guard/attack dog protecting the property, the only sign that you are supposed to have, by law, is one that says, "bad dog".
I sort of disagree ... I think a sign can help deter the more casual fruit stealer. If a potential thief knows that you care enough about the fruit to post a sign, he may pass not wanting to take the risk of dealing with an angry homeowner.
I do a combination of preventatives.
#1 ( most important) I have a fence around my entire property. If anyone has the balls to jump my fence to get a fruit, he/she has already passed the "casual" category. They have become a trespasser/intruder with no excuse.
#2 I have signs: Bad Dog, and you are being video taped signs.
#3 I have a couple of pretty big dogs.
#4 I have a video camera system.
If that's not enough, I "round" my property at least twice a day, making a visual check on my fruit trees. Vigilance pays IMO.
Another thing I do is have a talk with the folks that do my landscaping and take care of my pool. I tell them how much I love my fruit trees, how it is my hobby, and how I do not want them EVER to take any fruit. I also add, that if they would like something, just ask, and I would love to give them some fruit of MY choice.