If the increases are roughly the same, then linear is best. If the multiple factor is roughly the same, then an exponential is best. If the rate at which the increase increases is roughly the same (so A-B=1, B-C=3 C-D=5.2), then quadratic is best.
If the points have a roughly common difference, then you have a linear function. If the y-values are decreasing or increasing in a getting bigger or smaller way, you've got an exponential function. Lastly, but never least, if your y-values go from up to down or vice versa, you might as well have a quadratic function. I hope this all made sense and helped you.
