they paper trade. What i find really humorous is people here on ET who design systems, and then argue until they are blue in the face that paper trading results are valid if you follow the system!
They obviously don't get it that psychology and money management are prolly 90% of the battle.
They do get it after they trade for a few months and then lose a bundle however. Of course, they often don't know why, so they keep tweaking the system.