Help interpreting xinetd logs.

[CthulhuDragon]

I have been looking at my xinetd log file and most of it makes sense to me. However, under the telnet service I have += USERID. These never show up in the log file except one occasion where I got USERID: telnet UNIX: root. This was from an ip I did not recognize. I'm thinking this person was looking to do some mischief. How do I know if they were able to get into my system? Are there other log files I should check? Any help is appreciated.
Cthulhu Dragon