It's not hard to get the attention of the Secret Service. Write threatening letters to the White House. Or do as Russell Weston did--keep telling people you want to harm the President until someone calls the police. If all goes by the book, the Service is alerted, agents are dispatched, and the maker of the threats is interrogated and evaluated.
But the Service can do only so much. Unless someone is deemed a serious and immediate danger, he or she isn't placed under surveillance. Instead the agency compiles a file on the individual, entering the name into a computer. And unless...