Barry Rosenstein's Jana Partners disclosed a 7.2 percent stake in ConAgra Foods on Thursday and planned to nominate directors to the food-maker's board, the latest of many moves this year by activists, who have more money under management and bigger ambitions than ever before. Assets under management by hedge funds with an activist bent jumped $30 billion to a record $120 billion last year, according to S & P Capital IQ. Already this year, 90 proxy fights—shareholder votes for actions such as board seats and new corporate governance plans—have been taken up or are scheduled to be heard against U.S. companies at their annual meetings this year, already equal to the amount of disputes in all of 2014 and 2013, according to FactSet. So how can regular investors capitalize on the activist boom? An emerging trend over the last year—evidenced by Jana's move into ConAgra with a market value of more than $18 billion—is that no company is too big to wage an activist campaign against. Fourteen activist actions, like 13-D letters to management, involving large-cap companies in the S & P 500 were launched already this year, according to FactSet. These include Carl Icahn 's fight with eBay for board seats and Jana's other big move this year, a demand to Qualcomm that it spin off its chip unit. Assets "for activist investors [have] exploded in size in recent years, further putting pressure on companies to use their cash before someone else tells them how," wrote Jason Trennert of Strategas Research, in a note to clients this week. Read More Consumer finally waking up. Buy these stocks Once an activist makes a move, it's not too late to ride their coattails, a recent study showed. S & P Capital IQ looked at all activist campaigns going back the last 20 years. The research firm found a strategy of buying the day after an activist disclosed an action and holding one month out outperformed the market by almost 3 percentage points. If investors held the targeted stock for two years, they beat the market by more than 8 percentage points. So if the study is correct, the best investment may be to buy ConAgra shares Friday. Still, it could be worth screening for companies likely to be the target of activists in order to capture that often hefty one-day pop after a disclosure. Strategas Research sent a note to clients this week highlighting the S & P 500 companies that are not maximizing their capital returns by holding too much cash, a key trait that causes the activist sharks to swarm. Among those companies, CNBC Pro focused on the stocks on that list which are underperforming the market over the last 12 months, another characteristic that could trigger a move by activists. Read More As Fitbit soars, what's the top 'wearable' stock?