On the server,
type in this link in IE,
Then, to the left under folder 'Qlikview Servers', select your server entry.
Then, to the right, select tab 'Logging'.
Here, you can check appropriate checkboxes and also specify log directory.
Check the session log file in the path that Anju pointed you to, also please check Michael Terenzi's System Monitor that loads all log files into a QVW Application and does some out of the box reporting on them.