With the advent of the ARRL Logbook of the World, Internet log searches may become a thing of the past sometime in the future. I have now uploaded all of my 84,000+ QSOs (as of 11/30/2014) to the LoTW, and if you have also uploaded all your QSO's, it is a simple matter to check and see if you have ever worked me or anyone else in the LoTW. In fact you don't even have to do any searching as the matches are displayed for you automatically on the LoTW web site. You don't have to be an ARRL member to use the LoTW. If you'd like to find out more about the LoTW, click here.
If the LoTW doesn't suit you, I believe the next easiest way to find if you are in someone's log these days is to use the Microsoft Bing search engine. Just type the call letters of the station whose log you wish to check along with the word "log" into the search box below and click Search. In a split second a new window will open up with a link to that station's on-line log(s), if any exist. Or you can type his call, your call, and the word "log" to narrow it down even further. Another idea is to type your call and the word "log". This will show many of the logs in which your call appears.
Here are a couple of web sites that still maintain a list of links to on-line logs if you want to try them if the above fails.
Log searches #1 by QSL.NET
Log searches #2 by VA3RJ