The exchanger can be evaluated by first performing an energy balance to determine the heat exchanged.  I generally recommended you do this on both the hot and cold sides of the exchanger to determine whether there are any errors in the collected data.  The heat balance should be within 10%-15% if the instruments are fairly reliable.  The next step is to determine the effective mean temperature difference (EMTD) and utilize the simple equation Q = U * A * EMTD -- where Q represents the heat duty, A is the area of the exchanger and EMTD is determined by the fluid temperatures and number of passes in the exchanger.  Please refer to the book "Process Heat Transfer" by Hewitt, Shires and Bott (CRC-Press, 1994) for more information.