In BlueJ, go to the "Tools" menu and choose "CheckStyle". (If there is no "CheckStyle" command, see Installing and Configuring CheckStyle, below.) Note that this cannot be done if there are any uncompiled classes, so you need to do it after hitting "Compile". You'll see a new window labelled "CheckStyle". In the left panel are each of the classes in your program; clicking on any one of these shows you all the "coding style problems" that CheckStyle found in that class. These aren't compiler errors -- you can go ahead and run your program no matter how many style problems there are -- but they're things that you probably shouldn't get in the habit of doing, so I'd prefer it if you fixed all of these problems before turning in your homework.
You shouldn't need to do this on the lab computers on campus. For your home computer, ... fill this in!