Euklas provides error highlighting features (see #1) using error and warning markers that are similar to the Eclipse JavaEditor's highlighting features.
The checks for existing errors are more sophisticated if you work with a file that contains copied & pasted code. You will find more information about this in the section on Copy & Paste. You should keep in mind that Euklas employs heuristic analysis to check for errors that are not correct in all cases. The section Ignore Errors talks in more details about this.