In litigation, any question whether X was "reasonable" is typically determined by a jury, not a judge [0].
[0] That is, unless the trial judge decides that there's no genuine issue of fact and that reasonable people [1] could reach only one possible conclusion; when that's the case, the judge will rule on the matter "as a matter of law." But that's a dicey proposition for a trial judge, because an appeals court would reverse and remand if the appellate judges decided that reasonable people could indeed reach different conclusions [1].
[1] Yeah, I know it's turtles all the way down, or maybe it's circular, or recursive.