They're dead, so it's often a bit tricky to find out. What you could do is have some way of recording that people have been told about the opt-out, then they could carry a card around with them that says "I have been informed of the opt-out and choose not to, please take my organs". Only take the organs if they are carrying such a card.

And I'd be interested to know of the percentage of UK adults who are aware of the opt-out. I had a search around but couldn't find any surveys, are you aware of any?