By checking the log created within the session.Log 
indicates if any errors are present.

In Session method , after processing the session there is 
buttons called incorrect and correct.When we pressed the 
incorrect button we can see the errors occured during the 
process. And Log will give where the error occured indetail.

goto SM35 and there u will see log file.
if u get any error while processing data error record moves to  
log file and download the recors from log file and then any
reload the error records