I saw the show in Cardiff. I'll go over what I saw, and my thoughts - I'm only reporting what anyone audience member would have seen. No secrets.
During the interval, members of the audience had to ask ushers to be given cards to fill out. They weren't lying around.
The cards were in thick black envelopes with a white label for you to write your seat number and initials.
I didn't take part, but I stood near the bar watching a girl fill one out in front of me. If I was Derren, or a 'helper', I could have read what she was writing and taking in information about her appearance or even eves dropped her conversation - people can't keep secrets from their friends
Back in the theatre a queue had formed leading to the stage where the glass bowl was located. Participents were allowed, one at a time, onto the stage to put in their envelope and stir them around. I wasn't watching carefully enough at this point to see if envelopes were coming
out of the bowl, not just going in
The glass bowl never left sight of the stage, or was obscured.
During the act, he never opened the envelopes. The audience, including myself, assumed the envelopes were actually that of the names he called out. He never held up the envelopes for us to see the initials or the contents - he screwed them up and threw them to the back of the stage when he was done with them.
If he did use stooges, they were bloody convincing, and they sat with friends. He 'read' no more than 6 people out of the 150-odd envelopes. I thought it was many more until I counted them in my head.
One answer was "You have an allergy... a strange allergy... it's to a fruit... you're allergic to apples."
He then proceeded to just answer questions without the envelopes (which makes sense if you have already read them off-stage I guess).
Clever stuff.