In this case it's difficult to tell. A lot of small time acts hit it big with their one REALLY good trick they develop, or just good enough showmanship that there's reasonable doubt.

I've watched the show a few times and I don't know how carefully detailed the rules of success vs failure are, but on a few instances, Penn and Teller announced they had a fair idea of what tricks were being done but couldn't really place it in an acceptable order or say the how/when.

So even if Teller is the magic encyclopedia, he can be fooled, it seems.