Only way to cheat tax is for broker to give fake profit and loss statements to taxman. taxman cannot verify profits with exchange or banks or broker or have access to their computers. if broker colludes with client to cheat taxman it is very easy but the crime is 10 years in prison unless they shut down operations and leave country or close the business. this not an endorsement to cheat taxes but this is how some traders or hedge funds don't pay any tax on profits in the wall street casino.