How do you know whether a data set is a linear, quadratic, or exponential model?
There is no clear cut way to do this, but if a data set is clustered around a straight line, then a linear model is appropriate. It is a little trickier to distinguish between a quadratic model and a exponential model. Remember that an exponential function tends to grow faster than a quadratic function, so if a data is displaying a rapid growth, then an exponential model might be suitable.
I hope that this was helpful.