You can tell from a residual plot whether or not a model is linear.  If the residuals are uniform, then yes. If not, then the linear model does not fit.

For example, if you have 6 residuals and all of them are negative, then that is not uniform. If all are positive, it is not uniform. If half are positive and half are negative and they are about the same distance from the center line on the graph, then the linear model is the correct model.