If you've got the combat log; It's always displayed there with the roll...
It's chance - willpower/bodybuilding %
It sometimes deviaties from that though for some reasons, apparently the level difference mentioned above, but also some effects like rain seem to chance rolls more than they should.
Lastly, weapon effects (including things like blood from summon wolves) work the exact same way. Since the most I found on a weapon was 30%, all weapon-effects are basically worthless since they will never be applied unless the person has no willpower or bodybuilding at all... which is rare and only found on weak enemies anyway.
It also means pretty much all status effects from enemy weapons can be countered by a single point of willpower and bodybuilding, which I doubt is the intetnion...
Not sure if Working as Intended, sounds odd to me if it is...
