Take notice of the car they are driving. Police officers drive police cars. Now they can be unmarked, but they still have that look and feel of a police car. you wont see a cop pulling you over in a 1970 VW.
When you do pull over, try to do it in a well lit public place. A gas station or store front is perfect for this.
Ask for identification. Identification is harder to fake then a badge. It will clearly state their name and the agency they work for. Look out for a identification that doesn't contain this information.
If they are not in a police uniform then ask for a uniformed police officer to come. This is not a odd request and shouldn't be met with any resistance.
If something feels fishy then call 911. You will not get into any trouble for this. Remember that it is better to be safe then sorry.
