This is actually a pretty common misconception amongst muggles. A magician's trade is to lie with action, not with words, and most magicians I know have a very strong sense of honor in this regard.
Magicians do not often reveal their secrets, and I suppose some would even lie outright if you try to out them in public (although a good magician would find a way to ask you to stop ruining everyone's fun), but when a magician voluntarily tells you how something was not done, you can usually trust him. That's doubly true for respected magicians. I think it's a big part of what makes them respected.
Also, in the case of the bottles, cutting the bottom would not only be extremely easy to detect, it would also make the whole thing worthless. A cheap scam. Magician's consider themselves artists, not scammers.