I would say that a professional trader:
- Derives their income and livelihood from trading
- Treats trading as a business, not a hobby
Obviously those bullet points imply that you are trading money (yours and/or OPM) in such a way that you can live comfortably off the trading gains and that your business is structured in a way that makes the most legal and tax sense.
In the end, I think it's simply running your trading business as an actual business. And by that I mean you consult the proper attorneys, cpa's, etc on how to structure your business. The reason I say that is most 'non-pro' traders are never even going to get to this step due to losses and/or lack of desire.
To me, it's very easy to tell the difference between a pro vs. a non-pro. All you have to do is ask the right questions.