To elaborate on that one a little more... Dont just not pull over and dont just take off obviously just continue driving in a normal fashion... most police would advise that if you are unsure because it is unmarked then you should wait until you get to a well lit and very public area ie. a petrol station at night etc and as someone has mentioned, call the police and get them on the phone straight away, they will be able to check while your driving and assist you on what to do if your not sure.
Ive only ever been pulled over by an unmarked once before, but they showed me their ID straight away.