You learn to test your results against the most extreme case where the realistic answer is obvious. If your result fails the test, why then, oh yeah, there is something wrong, either with your model or your assumptions, so you change an assumption and try it again. If that does not help you start looking at the model, or for a wire that was not plugged in correctly.
Now not to pick on Prof. Tol, oh well, why not, but his work is full of failed bullshit tests. For example, using ones that have been discussed on various blogs recently, the recent attempted takedown of Cook, et al. Clearly, as Anon has shown, and as SkS has shown in a different but related way, there were obvious tests which would have shown that Tol was going off the rails.
And then the bunnies have Tol (2009), when Richard was told by Julie Nelson, that a positive value for Chris
And, of course, Tol's reaction to Bob Ward picking up on this. . .
Then, of course, there is the famous Ackerman Tol set to, where Ackerman and Muntz not only found an obvious divide by zero issue, but serious issues with Tol and Anthoff's calculations of agricultural benefits. The response on the divide by zero was typical Tol, we know about this, you would too if you understood FUND and we monitor for it. Given that no one else knew and this was not commented on elsewhere, a very weak response but made in full fury.
Another one was Tol's working paper on the advantages of accepting welfare and not working in Ireland. Obvious nonsense, and clearly so from the paper, but no, Richard has full faith and credit.
Be skeptical is good advice. Be nice, so when you make a mistake, and all bunnies do, others will be nice to you. Wait. . .