You can't and we aren't. What is considered rational, is often cultural and builds on very different epistemological systems.

Thus all you can do in a collaborative reasoning platform like ours, is give everybody the tools to portray their reasoning as well as they can.

E.g. an underlying reason for somebody's arguments might be the existence of God. An atheist would consider this irrational, but the logic, given the underlying thesis, might be sound. So in the end, we are going to debate epistemology. Something, I feel, we don't do enough.

Nor are we taught in school enough how (utility vs happiness) to evaluate the different epistemological systems.