Prediction with multiple regression

#1
I developed a multiple regression model, which I would like to use for the prediction. I designed the model based on a training dataset and applied it using a test dataset.

Now I have the comparison of my prediction and the real observation.

How can I now evaluate whether the predictive model is really suitable for forecasting? A first impression provides, according to the literature, of the mean square error MSE. However, this does not measure the relative effectiveness of the prediction. What else can I use to better assess the model? I am afraid that the MSE is too high for my model in absolute terms.

Thanks in advance!