Carefully written legislation
Below is an excerpt from the texts of the Communist Control Act of 1954. Note that membership in the Communist party could be discerned from the accused person's having knowledge of the purpose of the evil organization. Juries were instructed by this act (it established guidelines as to what was criminal behavior) not to limit themselves to evidence such as a current membership card. Here we go:
"In determining membership or participation in the Communist party of in any other organization defined in this act, or knowledge of the purpose or objective of such party or organization, the jury, under instructions from the court, shall consider evidence, if presented, as to whether the accused person …
… 8. Has written, spoken, or in any other way communicated by signal, semaphore, sign, or in any other form of communication, orders, directives, or plans of the organization …
12. Has indicated by word, action, conduct, writing, or in any other way a willingness to carry out in any manner and to any degree the plans, designs, objectives, or purposes of the organization;
13. Has in any other way participated in the activities, planning, actions, objectives, or purposes of the organization."