This a common question asked so don't feel bad asking it. First of all decide if you have been happy with the service you received. If you limo driver is on time, the limo is clean and the driver follows the instructions you have agreed upon then yes
you should give a tip. Here are some guidelines.
1) Calculate 15-20 % of the estimated cost of your trip.
2) At the end of the trip when you limo driver helps you exit the vehicle give him or her the amount you have decided on in cash when you say thank you and good bye. This does not have to be a formal presentation just say. Thanks for your great service here is a tip to show our appreciation.
3) Many limo drivers do this as a part time job and really appreciate the extra income.