lucy24 - 11:37 pm on Dec 4, 2012 (gmt 0)
If visitors got to your site by clicking a link, logs will tell you where the link was. But if they typed in your address or used a bookmark-- or if their browser doesn't send referers-- logs won't say what site they were on. If that's what you meant.
The referer (their spelling, not mine) is typically the second-to-the-last thing in logs: before the User-Agent, after the pair of numbers showing response code and filesize.