Emergency locksmiths do not take a deposit over the phone for a job. Payment is usually on completion.
Companies not in your area will give you a reference number (REF NO XXX) this job is then sub contracted to non specialists. Be aware they are likely to cause more damage to your lock/door, and charge more.
Check for reviews and testimonials on independent sites such as Yell or Google
A good locksmith will be able to give you a price range for the job from a detailed description of the problem. They will also check prices with you before commencement of any job.
Look for membership of professional body such as SAVTA or MLA.