The exchanges can track down every share that traded in a company's stock any day of the week, any time of the day. Investigators can contact the brokerage firms that were involved in any heavy trading, asking the firms to cough up information on any accounts that were active in the stock over a certain period of time. They can get their hands on an individual's account history and monthly statements -- even a person's address.
One red flag might be trades by several investors in one stock that all went through the same broker. Investigators can access a database of information on corporate insiders to match details with investors who were trading that stock. Someone's college alma mater might be the link between an executive and someone who was buying a company's stock just a few days before a takeover was announced.
Ultimately, exchange personnel have to decide whether to send a case on to the Securities and Exchange Commission. (The SEC can only bring civil charges. Waksal was charged with criminal insider trading.) And a case usually has to involve enough money to make it worth pursuing.
While it's easy to track someone's trading, phone calls and credit card purchases, the SEC ultimately has to prove why someone was buying and selling. And putting together a case can start with a single phone call. Two staff lawyers will call the person in question, with one attorney asking the questions and the other taking notes.And if human nature is any guide, the individual will agree to talk and then make up a story -- a dumb one.