To determine wether of not it is linear, you will need to see if it has a constant rate of change, if it does it is a linear function, if it doesn't it is non-linear. and to find the rate of change you will need to find the slope.
To find the rate of change, count the rise and run between two points. Then divide the rise by the run. If the rate of change is constant between all points, then the function is linear and it will be a straight line.
