@ScottF provided us with simple workflow that shows how you would set up a regression model and do an evaluation using the RMSE ( Root-mean-square deviation) => the lower the better. The KNIME numeric scorer node has you covered there.
You could switch out the type of model used:
We had a discussion about that here:
And also further information about ways to determine the quality of a model (mostly 1/0 but check out the link).
Further metrics I have explored are Correlation coefficients like Pearson and Spearman to see how your prediction and your real data do align.
Also I have toyed around with the concept of Bland-Altman plot but I am not sure yet if that gives me any more insight. Basically it should combine the question of correlation (if your score hints in the right direction) and agreement (if you actually hit the numbers).