I did a survey among 700 people in Tianjin and Beijing and was wondering how do I establish if this is enough to represent the population.

It really depends on how the research was collected. By this I mean how did you go about choosing which people you surveyed? The best way is through random sampling (that is the way using a list of randomly listed numbers). I know it is much more tedious and time consuming but it is really the only way to make sure you have a fair representation of the population.

The study involves obtaining auto insurance quotes and giving your opinion.

18-29 year olds…newspaper isn’t a good choice then

facebook
iTunes
email blast (even though I hate them 2% of people open them)
pay someone to fill out the info you want (www.microworkers.com)