Do You Know if Your Models Are Failing?

You're gonna tell if your models are failing by looking at some kind of fitness factor or fit statistic that comes out of running your model. Residuals in this context for definition are just the difference between your model and what the real data was. You can actually quantify that as a percentage. Look at the residual error on both the data that you just fit, and also in a cross-validation set, a holdout set of data. Run it on that and see how well, what the residuals look like on that.