Message Testing is the technique of trying out alternative messages on an audience sample, before you use the messages publicly.
First, you write several alternative messages. Second, you identify a sample audience group, generally called a focus group. Third, you assemble the group in person, show them your messages, and ask for their reactions. Or you can communicate with the members of the group via mail, email or telephone.
This is a powerful navigational tool, because one message can be many times more effective than another. In practice, it is common for one message to pull two, three or four times more responses than another message. In other words, this tool can give your program phenomenal leverage. It can multiply your profitability. However, this tool can't help you measure your profitability, as evaluative tools can.
As a marketing metric, it has three major weaknesses. First, it takes time, during the period when management may be eager to roll out the program. You often have to fight for the time to test. If your management is not marketing-literate, you may lose the fight.
Second, it can be expensive. (Some companies prefer to use mail, email and telephone contact in order to reduce the cost.)
Third, you have to be careful how you select and invite the participants, how you frame the questions you ask them, and how you analyze their responses.
Generally speaking, it's a good practice to enlist the help of a research expert, because there are pitfalls here that may not be obvious to the non-expert; for example, Nonresponse Bias and Dishonest Answers.
So, hire an expert.