For testing e-mail your client can use Infusion Soft, Get Response, Mail Chimp, Constant Contact, etc. All of these e-mail clients have their own version of split testing.
If they’re testing social media I believe HootSuite has a plug-in that’ll allow you to test various messages. You can test it yourself by just sticking content “A” out there on day one, seeing how many people respond. Sticking content “B” out there on day two and see how many people respond and what your readership is. Then you can start to get a feel for some of the responses and what’s going to work best and what doesn’t work.