This is an excellent article from Angie's List on the continuing difficulty consumers are experiencing avoiding scammers poising as locksmiths. Finding a local locksmith before you need one remains the #1 recommendation.

http://www.angieslist.com/articles/7-tips-avoid-locksmith-scams.htm

In an emergency I would suggest:

Write down who you called and the phone number.
When calling, ask for an estimate that includes ALL THE CHARGES.
When the technician arrives, get the technician's name.
Confirm the price you were told on the phone.
Ask if that is the total price and be clear on exactly what's included.

Do this before you let him touch your lock or door. You don't want to be left without a lock on you door if you suspect it's a scam.

If you think you are being scammed call the police.