Tell me this, then: what could anyone possibly do to prove that BS&T is not a Ponzi scheme? Describe a few more details of his business? Open up his books? Those can all be falsified.
This is simple. He just has to keep going without exponential growth of new deposits for a while.
A Ponzi only works with the influx of new people. Let it cool off, have a few people withdraw, see whether the money suddenly vanishes. If there's actually much less money in BS&T than claimed, it should not survive contraction. Growth of user base must be significantly lower than yield for a while, and some fraction of the investors should withdraw and not re-invest all the time.
The strange thing is that this hasn't happened on its own. BS&T looks like it has unlimited potential for abnormally fast growth, and that is suspicious in itself.