imho simplest way is probably by income and % income derived from trading-related activities
eg.
a) large mutual fund execution trader who makes 350k, representing 100% of his income
b) physical commodity trader who makes 1m
c) day trader in Chicago who makes 500k
==> all professional traders
as compared to
x) athlete who gets in on bitcoin and tech stocks a few times a year, makes 500k from it, representing 20% of his income
y) trust fund beneficiary who makes 800k from dividends and bond payments, not truly a trader in the sense that they are only holding onto a portfolio, but nonetheless still faces market movts
z) college kid who makes 20k trading penny stocks in the summer
==> non professional traders
ultimately maybe the designation itself of professional vs. non-professional is not as important as the actual increase in capital, because arguably (y) above is still doing better than (a) and (c)
