Regular investors and traders are the last ones to usually find out. The decision is taken in a vote and each member has to substantiate their arguments in writing, the details are eventually leaked to the SEC insiders a day or two before it is released to the public on the SEC website. The ones close to the source will have ample time to go short or long based on the decision. Influential whales pay good money to get such information before it get released to the public, and such an unregulated global market is probably the safest bet to indulge in insider trading.
Its kinda ironic that a free and open source software will become the plaything of insiders who will be the ones to profit, which ever way the actual decision goes. In any case, expect BTC to swing wildly as we near the actual date of the decision.