I didn't say COMPLETELY sure.
That's picking nits, I know...but you can't ever be completely sure. Reasonably sure, on the other hand...
Hell, let's just define it as "Sure enough that, if he pulls his hamstring again, the coaches will tell themselves 'Well, we rested him long enough, these things just happen' rather than 'sh--, we shouldn't have been so worried about starting five rookies on defense that we rushed him back.'"