One of the easiest ways to find expired domains with traffic is to look for backlinks. If a domain was developed and incoming links were put into place each one has the potential to bring in visitors. The search engines have made in harder to find those backlinks but it is still possible using a few tricks.
Previously you could do a Google search for “link:domainname.com” and get a list of incoming links. Although this still works it is unreliable. Even long time established website will have very few results using this operator. If you must use Google try searching for the domain name itself in quotes. This will show you all the instances of the domain name but you can narrow that down by clicking on a few results. An advantage of using this method is you will see unlinked but still listed references on message boards, forums, etc.
Since Google decided to change the way the list backlinks Yahoo has become the gold standard for finding them. You can still do a Yahoo search for “site:domainname.com” and get a list of the incoming links. You can even specify to include or exclude links from the domain name. Keep in mind Yahoo will limit your queries and if you attempt to bulk search they will temporarily ban you. Filter your domain list first to only the names you are most interested in and then search Yahoo manually.
Other Search Engines
Bing/MSN until recently was a good source for finding backlinks. However, as of this writing they have begun showing the same results as Yahoo making it unnecessary. AltaVista is another Yahoo product with similar restrictions. Your good choices are basically either Google or Yahoo.
There are services available that will gather the results from multiple search engines for a fee. While these will save you some time it is generally not necessary with all the free tools available. If you must use a paid service make sure it has updated results. There are some companies that have cached the results from prior searches and therefore may not offer updated information.
The days of bulk searching for incoming links are over unless you want to hide behind proxies which is not advised. The best method is to filter your domain list to only those you are most interested in. Once you have a short list take the time to do a manually search on Yahoo for linked results and Google for forum mentions. Although you will probably not get an exact list it will give you a good idea of what is out there.