Perform a surface scan

This option is available from every CD/DVD or (HD, Flash card, ...) Device icon.

In other words, make sure the drive is selected via the drop down menu in the top left corner, so that you can see the content displayed in the left and right panes.
Next, in the left pane, select the top most CD/Flash/HD icon, right mouse click and select "Perform a surface scan" 

After this scan, IsoBuster will inform you of the amount of errors encountered.

Errors on a disc/disk are not necessarily always a problem.  They might be located in areas where there is no useful user data.  

For instance on optical media (CD, DVD, BD) sectors that are 'often' unreadable, depending on the media and the way the tracks were written :
- The last two run-out blocks of a session
- The run-out, link and run-in blocks (7 blocks) between every packet (in case of packet writing)

After the scan, in case there are file-systems with files on the disc, files that are located in erroneous sectors will also be 'marked' with a special icon :  IsoBuster - Perform a surface scan

The list with erroneous files can always be recreated without the necessity to redo the scan.  Just select the File System icon and create the list via "Directory tree and file information".