Trading volume normally will spike if large institutions place a big move. Along with the stock price's change, you can make some guesses. For instance, if trading volume spikes relative to the stock's average volume at the moment when the stock price also goes up, that may mean large investors are buying. Alternatively, if you notice a stock's volume increase and the stock price drops, you might guess institutions are selling.
The drawback of this method is that volume can be impacted by external variables, like options expirations, short selling and other trading sounds.
There is a better way than simply investigating volume to determine when there is institutional buying and it is referred to as the $TICK chart.