You cannot guarantee that users will read text.

Assuming you put it front and center, you can help them *notice* the message if you **keep the text short and to the point**. More text is dilution. People scan, they don't read. 

A/B testing doesn't lend itself to all decisions but it's perfect for copy testing. If you wanted to compare 2 writing approaches to see which is more effective at getting users to notice or comprehend certain ideas, try a few experiments with [memory tests.][1]

Many ways to test assumptions, this is just one. 

  [1]: http://verifyapp.com/m/23374