1. DON'T LET PRICE BE YOUR ONLY CONSIDERATION
After shopping around for the best price, Federal Motor Carrier Safety Administration spokesman Duane DeBruyne recommends looking up safety records for companies that are authorized to speed passengers across state lines on this federal site: http://fmcsa.dot.gov/safety-security/PCS/Consumers.aspx
2. CHECK WITH YOUR STATE AGENCY TO MAKE SURE THE COMPANY IS FOLLOWING SAFETY REGULATIONS
For companies that only carry passengers within one state's boundaries, check with the agency that regulates chauffeured vehicles about whether they're up to code. That could be the state department of transportation, highway patrol agency, state police, or as is the case in California, the state Public Utilities Commission, which maintains its own database: http://tinyurl.com/CPUCdata
3. MAKE SURE THE COMPANY YOU PLAN TO HIRE HAS INSURANCE AND IS LICENSED TO OPERATE
If the state agency can't immediately provide you with the information you seek, ask the company to provide you with proof of insurance and their license to carry passengers.
4. MAKE IT PERSONAL
If you like the way the company sounds on the phone, the National Limousine Association's executive director Philip Jagiela recommends stopping by the company's offices in person to see what you think of their fleet of vehicles (and particularly the one you will be renting). His association also maintains a list of its members, which have gone through an application and vetting process: http://www.nlaride.com
5. GET IT IN WRITING
Ask the limousine company to give you a written agreement with the cost, deposit required, refund and tip policies, pick-up/drop-off times, arrival and departure locations, type of limo requested and specific services you expect.