If you mean in Event Viewer, you can do one or more of the following:
  • click on the User column to sort it, then scroll to the find the user you are interested in (rather than scroll, you can use the next option).
  • right-click on "Security" and select "View"->"Find" to search by user (and/or whatever else you like).
  • right-click on "Security" and select "Save Log File as.." to export to a text file, which you can use Notepad to open/search.