How to investigate the masking logs varies based on the specific version of the masking software you are using.
For the fast-stack versions, you would look at whatever base-directory the software was installed in and from there go to the logs/AppLogs directory. The dmsuite.log files are located there.
For the stand-alone engine, you also log into the underlying OS and go to /opt/dmsuite/logs/AppLogs.
For the standalone engine, you need to log into the GUI, select the "Admin" tab and then "Logs" from the menu on the left. Once you do this it will bring up an extremely simple web file viewer in the GUI which will allow you to traverse the logs in dmsuite.log. Much more versatile, however, is to use the "Export" button in the upper right to copy the file to your local machine and then you can use whatever viewing/searching application you are most comfortable with.
The in-browser viewer is there for convenience, but it is not a very comprehensive, full featured text browser.