Toughness is difficult to quantify. Pain thresholds vary, and some team doctors are more lenient than others.
Yet for NFL quarterbacks, toughness is actually very easy to judge. It comes down to one question: Can he take the field when his team needs him?
It's normally not a good idea to distill a broad subject like this down to just one stat, but in this case, there's only one stat that matters.
Could he consistently take the field every single week?