Fedora Security :: How To Tell If Computer Is Being Attacked

[URL] This web page says: "The team set up weak security on four Linux computers with Internet access, then recorded what happened as the individual machines were attacked. They discovered the vast majority of attacks came from relatively unsophisticated hackers using "dictionary scripts," a type of software that runs through lists of common usernames and passwords attempting to break into a computer."

How did they "record what happened as the individual machines were attacked"? How did they figure out that "the vast majority of attacks came from relatively unsophisticated hackers using 'dictionary scripts'"? What I am really getting at is that I've searched the net and found lots of advice on how to detect if your computer has been hacked but I haven't found ways to know if your computer is being attacked. Obviously, this group did that.