Identify the warning signs
Unreasonably low prices
Scammers are everywhere and Sonoma and Napa county have their fair share of them. When the price seems too good to be true, it probably is. Please don't fall for the $19 dollar locksmith at 8pm at night. They want to just show up and break the news how much more the work will be.
Answers the phone with generic "locksmith services"
When they don't have a real company name then you should just hang out. There is no company named "Locksmith Services". That's like calling a bank and they answer with "Bank Services". Any reputable company will identify themselves as their actually name.
Negative reviews online
Bad reviews are a huge red flag on what kind of service you will be getting. If you're not sure what kind of reviews they may have, it's worth a minute to check their Yelp page.
Arrives in an unmarked vehicle and plain clothes
Calling someone out to your home or business can be very personal. You need to beware of who you let in your home or trust with your keys. Don't feel bad to turn them away. If they are unprofessional then they are asking for it. Check his locksmith license and see if it's valid. You can check licenses at http://www.bsis.ca.gov/
Destroys the lock to gain entry
If he suggests breaking the lock to get in, there better be a really good reason for it. Some locks CAN be high security which requires forced entry. But 98% of the locks out there do not need to be broken. If he's a real professional then the lock can be saved 98% of the time.
Only accepts cash or insists on cash upfront
Again, if we are putting our trust into their company, then they should trust that we will pay them as quoted. Additional costs from your initial quote should be explained to you before work persists. Do not except a larger bill if they did not inform you in the beginning. Real professionals should be able to accept other forms of payment than just cash. Get a receipt!